I looked into this even though I have no skill. I turned several of my computers int hackintoshes. so I thought iOS might be possible.

Here are the problems I remember (it's been several months so this is bound to be off):
1. A whole lotta closed source.
2. Tightly controlled hardware. This is one of the big deals because apparently MacOSX uses kernel extensions to kind of load drivers, but since iOS is designed to run on just a handful of different components they were able to put it in the kernel without all the kexts. Or something. Hopefully you read my earlier disclaimer about not knowing much at all about the nuts and bolts.
3. ARM and Darwin. Apparently there are only a handul of people on the world who have the expertise to do this. Apparently they all work for Apple.

I think there were other reasons. As for the peope who ask, "Why downgrade your phone?" They are completely missing the point. Why climb a mountain? Because it's there.