uhhh Crysis actually does look better in DX10. Still runs like fuckpiss, but it does look better.
Only because the "Very High" mode was artificially disabled under DX9. By editing config files you can get it to look exactly the same.
Otherwise, there just isn't big enough of a difference to market Vista. By disabling effects under DX9 in new games, it encourages more people to upgrade to Vista to get them. There may be requirements in the "Games for Windows" program to give a certain number of extra features in the DX10 version, and developers take the easy way out and just disable features in the DX9 version.