Because the Casual market seems to bring in more profit than the "Hardcore" market.
they're not really listening though, if you're intelligent enough to install a pc game, then you're intelligent enough to use dedicated servers, find out about them.
I cant see their reasoning behind it.. really cant.
The problem is also that it sold like 1 million copies on PC and 8 million on xbox 360. As a company you know which group you are gonna give the highest priority.