My opinion :
1) Apple aimed at this result. And a long time ago. The underlying OS for all devices is the same (almost). So the development platform is kept tight and clean, only building up with time. So is Microsoft and Google. And them only, the rest is decorative (I know Palm, RIM,...). Look at bottom line.
2) Apple is in the business of selling software. People buy into MAC because it's EASY probably a bit more than beautiful. Nokia (Intel, others...) are about to be... This is true all the way to devs.
3) Choice. People enjoy it by nature but to get work done...what you really need are leaders(to make choices) and followers(to really make it happen). I'm sure all Apple/MS/Google ruling bodies know "Divide et impera". And they even know better to keep their enemy closer (how good does Google make the i-thingies...?). So Linux world is left with conceptual fights and unfisnished wikis...while AMG enjoy making business.

and finally...
4) You can't compare Apple and Linux. They don't aim at the same stuff. Apple is business, Linux is the universe. People buy a mac or i-thingy, especially "the new or last one". People CHOOSE Linux. To be or to have....
So by nature developers that want/need to HAVE money will choose Apple and get what they want (and quick, and easy, and more...) whereas dev that CHOOSE Linux will only get what they are looking for (could be anything though, including there is no spoon! ).

Me? I wanted something different, and I GOT IT .
also...i felt like writing this...