Serious question. Not trolling. I walked into a Best Buy and was over by the iPads and some guy just started saying a bunch of negative comments about the iPad and how it's not good for anything because you can't install your own apps on it. There were a lot of other things said, but I just walked away.
Putting into perspective what the Best Buy guy said, I would say that as previous stated, the major reason is the iOS Ecosystem and limitations. Some people want to: Use a different App Store, have a SD Card Slot, have a universal file system to store stuff, customize and install various widgets, keyboards, etc... I could go on but these are all typical Android vs. iOS topics.
Of course there are tradeoffs since the iOS Ecosystem gets a lot of things right.