There are two pretty distinct reasons.
The first is for OSX lovers: They do it for the reason that Bugbyte said in post #2. Windows dominates the market so there are naturally some things that don't get ported. There are emulators out there, but they try to convert the code and it boggs it down to not usable.
The second, is for Windows lovers (like me): I love windows because to me it is more intuitive. I don't like the graphics but then you can make em look better yourself! Well the reason to do it is because Apple has done something very very right aesthetically. They
look good. My PC like most others' here, you built yourself. They look good because of the case and unique-ish look, but c'mon... Macs look x10 better. I like the sleek look that a PC really can't achieve for the same pricepoint. I refuse to use OSX though, so I shrug and say mines good enough. Now though, you can put the best of both worlds together, and run Windows, on a sleek Mac. The best example of sleek and stylish (other than iPod), is the Mac laptops. 5 hour battery life, thin as nothing, absolutely no fans, beautiful screen, and now your choice of Windows. That seals the deal for me...
