Possibly because x64 OSes aren't as mainstream as x32 OSes at the moment. It's like the old browser wars - people claim IE has more vulnerabilities just because it is the dominant browser. I think same could be said of OSes.
I have not seen working x64 viruses, so I never used any particular anti-virus software.
Edited by TonyW, 15 March 2007 - 02:17 AM.