The biggest factor for PC development is it's cheap and easy. Really cheap. Much cheaper than on consoles in fact. PC development is so much easier than console development that games like Bioshock and Devil May Cry, which the developers don't expect to sell anywhere near as well on the PC, are being developed for the PC then ported to the consoles (the primary sales versions). This is because it's far easier and cheaper to build a game on the PC then port it to one (or both) consoles than it is to build a game for one console and port it to the other.
The data at the beginning of the thread is incomplete. It's missing many of the million selling PC games such as Warcraft III, Return to Castle Wolfenstein, Starcraft (and expansions), Everquest, and about 100 others. It's simply very very flawed at the moment.
The final, and most important point, I'd like to discuss is digital distribution. Much of a game's sales now are done digitally. Steam and other digital distribution services make up a very large portion of PC game sales now. Valve reported over 70% of Orange Box's sales were made on Steam.
This is a huge deal folks. First of all, these sales aren't tracked by anyone. Second, nearly EVERY dollar here goes to the developer. Think about that for a moment. A $60 Xbox 360 game makes what? About $25 for the developer? At most? A $50 game sold on steam makes $50 for the developer. There is no retail markup. No huge publishing fees.
You people who think that the PC market is not a lucrative market are missing some really major factors. The PC software market is hardly dead. In fact PC software sales have experienced significant growth over the last few years according to Gabe Newell in a fairly recent interview.
