Crytek no doubt assumed that by the time Crysis came out, Vista would have a big market penetration. So perhaps they coded the advanced graphics with DX10-exclusive language that they now can't or won't remove. No way for us to know.
What we do know, and they will soon find out, is that it's taken on political overtones just like BF2142's surprise insertion of ads. BF 2142's move led to it underselling and is now in an early grave. We'll see if Crytek can salvage their mistake any better.