There have always been plenty of games only available for one platform or another. I suspect what is changing really is that a lot more console games are now looking desirable to the PC player than ever before, while the increased complications of cross-platform builds have made ports less likely.
PCs still have utter dominance in terms of RTS games, but in most other areas that's no longer true.
There's also the piracy angle - or rather the perception of the piracy angle. Rightly or wrongly, a lot of games companies think most of their sales will be lost to piracy, which makes the PC format less attractive.
