So why aren't linear trackers more prevalent? You'd think with the tech available today and potential convenience and ease of use they'd be brilliant. Vendors like the margins on pivots and audiophiles are fearful of messing up their records? Or is it the constant coupling of the tonearm to a gear or motor?

A good linear tracker is not as easy as you might think. Fremer has written about the potential problems with linear tracking arms on numerous occasions. For example:

"One problem facing designers of linear arms is how to drag the arm's mass laterally across the record surface. Another is how to route the wires from the sliding arm to a fixed termination point without impeding the arm's freedom of movement. Noisy, tweaky, motor-driven servo systems have been used on a number of linear designs both ambitious (Rabco, Goldmund, B&O) and low-tech (plastic turntables from many mainstream manufacturers). But even the best of these require the loss of tangency to signal the servo correction system to move the arm along the track. The correction inevitably overshoots or undercorrects to some degree; the result is an arm that wiggles its way across the record, creating potentially greater tracking errors—and more of them—than you'll find with a properly set-up pivoted arm. The ideal is for the cartridge's cantilever to maintain perpendicularity to a radial line drawn across the grooves. This can probably never be achieved by servo-type linear designs, even if, from afar, they look as if they are, and even if many audiophiles remain loyal enthusiasts.
An upgraded iteration of the Souther linear-tracking arm, built and currently marketed by Clearaudio, uses a different scheme. A very short, low-mass rod rides on wheels along a pair of low-friction quartz rails. Discussion of the design tradeoffs is best left to a full review, but I remain skeptical about this arm's ability to maintain true tangency to the grooves as it rides the rails.

The virtually frictionless air-bearing designs originally produced by Maplenoll and Eminent Technology were both designed by Eminent's Bruce Thigpen. In these arms, the bearing is fixed; it's the rail that moves, with the armtube attached to it. Eminent still sells its ET2 arm; a heavily modified edition of the original Maplenoll design can be found on Walker Audio's Proscenium Gold turntable.

Finally, a number of air-bearing designs use a fixed rail and move the bearing, which has the considerable advantage of moving a far lower mass across the record surface. Included among these are the Versa Dynamics and Forsell (neither any longer in production), and the Airtangent, Rockport, V.Y.G.E.R., and, now, the Kuzma Air Line. These designs vary in bearing mechanics and air flow, as well as in overall build quality and ergonomics.

In conventional pivoted arms the arm/cartridge system moves vertically and horizontally around a common point; thus, horizontal and vertical effective masses are very similar. In linear trackers there is a big difference between the effective vertical and horizontal masses. Being a pivoted system in the vertical axis, a linear tracker's effective vertical mass is low because it consists of the relatively short armtube and cartridge. Horizontal mass is much larger: it includes the entire arm/sleeve assembly as well as the cartridge, all of which must be carried across the record and which do not benefit from being a pivoted system.

"Hang a small weight on the end of a spring and it bounces at a fairly high frequency over a short distance. Put a bigger weight on the spring and the rate of movement slows while t he excursion length increases. The high mass of a linear-tracking arm in the horizontal axis can create a very nasty low-frequency resonance. The eccentricities due to the off-center pressing of virtually every LP made will excite this resonance as the system moves back and forth trying to track the shifting groove."

The Kuzma sounds absolutely incredible, but if somebody gave me the choice of a free Durand Telos pivoted arm or a free Kuzma Airline, I'd take the Durand. It's one of the best sounding pivoted arms in the world, the best that I can remember hearing at least, and using the Kuzma is a giant pain in the ass. Tracking errors from a properly set 12" arm are minimal, and the annoyances for an air bearing linear tracker are IMO not worth it.

I respectfully disagree based upon my personal experience in setting up AND using the Kuzma Airline tonearm.  It is a worry free and trouble free experience, a real pleasure to use as it is just so well engineered and built.  I am saying this after setting up the WHOLE turntable from scratch, setting up the pump/compressor and ensuring the proper air pressure at the tonearm end.  It is absolutely NO trouble to use at all.

I will have to check out the Durand Telos at some point...if I like it, damn....I know where this rabbit hole goes! In through my wallet and out the big hole at the other end!

My experience with the Airline is admittedly very limited, but from what I recall, it was incredibly finicky, didn't like records that weren't uniformly thick, and required constant, constant re-leveling. Basically, play a record, re-level. That's just not something that I'm willing to deal with on a regular basis. I suppose it's possible that something was off with the setup and that's what was causing it to misbehave more than it should.

As far as pivoted arms go, as I said, I have yet to experience anything better than the Telos. It's incredible, and I would put it up against any arm at any price, regardless of bearing type or design. Graham, MØRCH, Da Vinci Audio Labs, and a few others also make exceptionally good pivoted arms, and some of them do certain things better than the Durand like VTA adjustment, but sound wise I think it's in a class by itself.

Wood arm...most curious...

Yup. Joël Durand started making arms for himself by hand, and wood was the easiest thing to make them out of. It's funny, his approach is pretty much the exact opposite of ultra high-tech carbo-tanium or other super fancy composite materials that other companies use for their arm wands, and yet the end result is still better. He was asked about it in an interview:

"At first, it was just a practical matter. Because I started with such primitive means, wood was the easiest material to obtain and work with. I also early on developed the idea that the arm wand’s role in the transmission of mechanical forces was fairly similar to the role of the bow used on string instruments. I researched musical instrument construction and discovered some interesting similarities that reinforced my decision to use wood. I’m regularly asked why I offer only one type of wood for the arm wand. It’s simply because it’s the one that gives the best results for what I want to hear, and considering the rest of the whole tonearm system. These of course are two very significant reasons: I know what I want to hear, and I don’t claim that my choice is the best one for all situations. The wood I ended up choosing happens to “mate” extremely well with the metals I use in the base, the way they are shaped, etc. It might not work as well in another design, with different materials, who knows? Another essential aspect of the arm wand is the wood finish—a mixture that I make myself. It’s used for the protection of the wood and to maintain its stability in various atmospheric environments, but it also has a very important impact on the sound.

Going back to the question of the wood itself, I had a number of meetings with professors in the department of Materials Science at the University of Washington, because I was curious to understand exactly why the woods I had experimented with had such different sound characteristics. After analyzing them, I discovered which parameters were important (and which were not so important) for their use as arm wands, and verified that my intuitions about the similarities with instruments were correct. I am not committed forever to the idea of using wood, though. I’m still experimenting, and if I find materials that perform better, I’ll switch."

On the new Technics tangent here is some info direct from Panasonic about the new motor:
http://news.panasonic.com/press/news/data/2015/09/en150903-3/en150903-3.html

Kind of looks like they are going for an updated SP10 with a badass motor and speed controller like the VPI DD.  I'm hoping the Panasonic economics of scale will get the pricing down to the 5-10K level (with performance like the VPI DD  ) because that would be all kinds of awesome.