I think that, like with the ipod, it's just because they've managed to make a name for it on the market.
I wouldn't trade my MDA Compact for anything, phone sound quality is as good as with any "traditional" mobile phone, software is not entirely bug-free but generally works well, I don't really use the extra features like the camera but they're there if you want them, and the battery life is surprisingly long!
On the other hand, I've heard horror stories about the A1000 and other devices and people like to "go with the flow" and buy a widely recognised device that's accepted as reliable (and/or the latest business accessory).