Not that I think macs are useless or that people who use them are stupid, since they seem to be targeted towards people who aren't exactly tech-savvy, but I think it has something to do with them costing too much, having worse hardware, not being able to be repaired or upgraded, or being incompatible with lotsa stuff.
-I've never had a mac, this is what I've heard from a friend who recently spent $1200 on a macbook pro. It was on sale and he thought he could use it for running X code to make apps and stuff. He tells me his favorite thing about it is that it runs windows.
I think it's kind of funny how my $200 dollar laptop/large netbook I bought 2 years ago is almost as good as a newly bought $1200 mac- we only compared minecraft fps. Mine gets 25, his gets a little over 30.