Because Crysis at most can only utilize 2 cores. It uses one for the game and one for sound hence why one of your cores tends to spike a bit.
Also CryEngine 2 is horribly optimized and not very multicore friendly hence why a dual core at a equivalent speed of a quad core will get the same FPS.
If you want to see a game using all of your cores, try Left 4 dead. I saw it use 80% on my Q6600 on all 4. Probably less on yours since it has 8 threads though.