Microwaves go in the kitchen. I would suggest that it is up in a higher spot so kids can't play with them. That is a good reason they are often over ovens along with the venting.
Also from an electric point of view almost all microwaves require a 12A dedicated circuit. For a kitchen you will probably have several runs like this and it is just bundled with those. For a bedroom you would be basically running one circuit into a room for one outlet, where the rest of the room is on another. Just a little odd.
From a safety stand-point microwaves draw a lot of current. I guess there is more of a chance (albeit very low) that microwaves cause more fires due to current fluctuations than most other things. I would much rather a fire in my kitchen than my bedroom or anyone's bedroom.
And practically speaking I remember living in the dorm and my roommate burning popcorn in the microwave... Good luck falling asleep with burnt popcorn smell.