Take your pick as its some of the following:
1: PS3/X360 account for approx 70-85% of multiplatform gamedesign sales. PC struggles to even hit 10% unless its an RTS,FPS or PC game design.
2: Piracy on PC is very severe so Publishers are not as bothered about the PC anymore as X360/PS3 account for the vast majority of sales. COD4 according to IW was pirated over 60%
3: PC hardware config minefield means it can take longer to get the game running on a reasonable spec PC or complex game breaking bugs can be hard to fix.
4: PC @ retail is dying quickly and now the royal mail strikes are hurting PC gaming even more as suddenly its no longer attractive to pre-order games unless you pay a lot more for special delivery which is no longer a guarantee to even get them so you have to look at the retailers and many are no longer stocking PC games or do not get the latest titles until several days after the release date.
Its actually easier now to port 360 games to PC as Nvidia/MS/Intel all have developer tools which translate the 360 code into PC friendly code very quickly. If PC games consistently sold even 25% of the console version you might see them come out much sooner as PC has a potential market over 250M but also consider that the same developers might have to finish the console version before switching over to the PC version.
If all the fools who pirate PC games bought them the PC would be much more than the niche gaming platform it has become. Right now PC lags handhelds like DS & PSP so its probably close to last in sales number for multiplatform game designs. This is so pathetic considering there are more gaming PC's in the world than X360+PS3+PS2 combined
