Doesn't anyone remember, back in the day, when HTML was "just fine" and javascript was 'clunky', 'unusable', and had a multitude of accessability issues. Cross-browser javascript problems, anyone?
Now, of course flash also has its issues, but believe it or not, they are, much, much fewer and smaller than javascript/ajax had when it first started out, and even, dare i say, now.
The accessability issues, associated with flash are usually (as always) because the developer didn't know how to fix them, or didn't care to. There simply hasn't been a "killer app" like gmail, to show all the world that it could do thing much better than is generally asumed.
For example did you know that you can make a flash site change it's url as you navigate through it, enabling you to link/bookmark a specific section or a page of it (also allowing you yo use back/forward buttons)? The accessability issues have also long been solved, giving 'no-flash' users a striped down, yet fully functional/accesible site (as is the idea of javascript behaviours, if I recall correctly). (Take a look at this site http://www.rizn.bg for example of the two above - I know it's not english but the important thing is the concept)
And flash in itself has much more potential than javascript/ajax. 3D animations, build-in optimised compression algorithms, easier server integration, video/audio streaming, webcamera support, better file upload, etc, and it's all build in, on a single platform, with a quite nice and consistent api on top of it. Actionscript (the flash language) is a quite nice blend of java and javascript, minus the java bloat, complete with classes, inheritence, true C-like data types (insane performance), clusures. And it's all a lot more cross-browser friendly, than any other competing tehnology.
Now, i'm not suggesting we all jump in, because it does have issues still (font rendering, os integration and of caurse performance, to name a few) but there are quite a few places where it is a very good alternative. Just because most of the flash we come in contact with are useless splash screens and tacky animations, it doesn't mean that it can't do better. It's a hugely undervalued platform in my opinion, and we kinda have to 'wake up' and talk about its benefits instead of "it'll never work, its better to stick to what you know".
As for the original question - maybe because when it started out, flalsh was not good enough (pre flash 9 time), and by now they already have an infrastructure in place. Maybe if they create something radically new, they'll try to adopt flash, otherwise they'll stick with what works for them now.