I don't know about you, but except for a C++ class in my sophmore year every single computer in my school district was an Apple system. And I'm sure quite a large majority of districts use Macs as well.
It was only once I actually had to do things that mattered that I really got into using Windows.
I'm not an average user, have some odd peripherals, and like to play games. Thus, Windows and decent hardware.
And considering you can get a decent PC for 600 or so now WITH Windows...
As to OSX that's because they refuse to sell the entire OS standalone and support the open-hardware environment.
Ubuntu does not do enough to make me happy.
If you want to get technical, Apple-, MS-, and Linux-centric hardware are all PCs. Especially now that Apple runs on x86 and the hardware itself doesn't matter.
Mac/PC ad freezes, nerdy looking guy walks in front.
"And hi!, I'm linux. I'm free, unlike these guys. And I've been configured to run on just about any hardware!
"As long as you don't care about using all the software these guys use, and are willing to deal with no real customer service, I'm a great alternative! I admit that unlike with these guys I don't easily work with the hardware you already have...
"Ummm...I'm really best in a server...en..vironment..." (trails off)
Walks off with head down.
You can be replaced by this computer.