That may account for a small part of it, but there are millions and millions of Macs out there. Scoring the first virus hit on them would make a very tempting target for the malware scum.
However, I think most malware attacks have gone beyond the nerd hobby boyz. Most of it seems run by organized crime and hostile governments, these days. More profitable to target institutional users than individuals. They'd much rather have a keysroke logger in a bank clerk's PC than some Art Director's Mac at an advertising shop.
"Scoring the first virus hit on them would make a very tempting target for the malware scum."
First ? from 2004
Destructive OS X malware spies on Apple users
http://www.zdnet.com.au/news/security/soa/Destructive_OS_X_malware_spies_on_Apple_users/0,130061744,139164062,00.htm
... and each quarter there are more PC's sold then the combined total of every Mac EVER sold.