Any ideas as to the reasons the JD-Xa isn't selling well? It looked like a decent instrument to me, except for the exterior body.
No idea. I've only heard from a couple of retailers that it sells worse than the Boutique series, which in turn supposedly sells way better than Yamaha's Reface series.
It could be the toy-like appearance, it could be the at times dated sounding presets (which mostly reminded me of 1990s techno stuff), and it could also be that it seems to complicated to some. With regard to the latter there are parts of the synth that aren't so easy to navigate (cf. Nick Batt's review), especially if you want to start from scratch.
In general, and especially with the analog revival in mind, it seems to me that most buyers are especially interested in instant gratification instruments. Sound designer's dream machines are not really for the mass market I suppose. They might get good reviews, but are not necessarily everybody's darling, as not everybody wants to dig deep into a synth's architecture. Look at mono synths for example. I, of course, would expect to see way more Pro 2s out in the wild, but if you look at festival stages or whatever, you still see a majority of either vintage monos, MS-20 Minis, or Moogs. Or think back to the Evolver for example, which to this day would be a super affordable and hard to beat synth in its desktop or MEK incarnations. (I still know plenty of synth heads who have absolutely no idea about the Evolver.)
So I guess it is probably related to a certain degree of conservatism and maybe a lack of courage to try out something different here and there. But these are just my personal assumptions.