I assume one of the big main reasons is because of the limited hardware capabilities of consoles. Developers need to cut corners here and there in order for it to be optimized and perform well on a console. You'll notice Crysis is a PC exclusive, where as Crysis 2 is not.
I've just noticed that Crytek put Crysis 1 up on PSN and Xbox Live last year, but I can guarantee it doesn't have the same level of quality as the PC version.