http://trailers.gwn.com/movies/play.php/f/535/Crysis-Crytek_CEO_interview.html
Here. A Crytek guy explains why they didn't make it on PC (about 3/4 way in).
He says:
1) The engine was multiplatform, but they decided not to make Crysis multiplatform.
2) They wanted to prove that the PC was not "retracting" and can "kick ass".
3) The gameplay and the complexity is only possible on PC. A console version would have to be stepped back.
4) They played around with it on console, but they would have to compromise things like having such "big worlds".
5) They may go console in the future, but didn't want to be distracted in Crysis 1.
Then he talks about really wanting to push licensing the engine. They made this amazing engine with amazing development tools, but it's not getting many licensees. I think a big motivation for making Crysis 2 multiplatform was to show off what their engine can do.