The reasons are clearly either politics or incompetence. Now, I'm not sure what those politics might be. I don't personally believe that its that Adobe is "angry" at Mac or something, but I for example know that they are absolutely paranoid about accepting changes to the core code (even obvious crasher fixes require a ton of approval and consideration).
It really is to their own detriment. Regardless of how big the Mac market may be (its too big now to ignore and getting bigger), its a very powerful market of people that matter: i.e. developers. If Adobe got their act together, they would wipe the floor with JS, HTML5, etc. I mean, they almost do now, its purely due to their inability that they haven't completely and absolutely taken over the web.
Of course, the same can be said of standards. Taking 10 years to have any sort of measurable change on the W3C side is equally unacceptable (and we still ended up with a video solution that guarantees Flash will dominate this field for the next 10 years). Its always funny to me when people talk about whether "standards" or "flash" is winning, because it seems like a race where both parties are running backwards.