The problem with PC games is they have no standards enforced on them like console games do. If you're making a console game, they have to pass certain requirements enforced by Sony, Microsoft, and/or Nintendo (the game has to be submitted to each console you want your game on). If your game is being put on PC, there's nothing that checks to make sure they pass those certain requirements.
If the game is something like a 360/PS3/PC game then the big stuff will at least be handled by the 360 and PS3 submissions but since PC games don't generally make as much money as console games anyways, it's a crap-shoot if the developer actually puts a lot of time into making sure everything is hunky-dory on the PC.
Furthermore, while there are only a few different PS3, Xblx, and Wii configurations out there, there are an infinite amount of PC configurations and those all cannot be tested. You can hit the big ones but, again, with the PC version of games bringing in a much smaller profit than the console version, there's not a large incentive to test the game on many different configurations.
I know PC enthusiasts love theirs PC gaming and think it's the best, but PC gaming is really the red headed stepchild of the game industry and is becoming less and less significant. It's just easier to make console games and it's easier to make money on console games. It costs too much money to thoroughly test a PC game and the return is just too little.
(and yes, I realize there are exceptions)
