Mac OS is unix-based which was designed from the ground up as a multi-user environment, whereas windows, especially before XP was designed as single user.
Having a multi-user environment is generally more secure because each user only has specific permissions (no self respecting mac user would run everything as root) and is unable to change many system files. In the small chance that a virus is executed only the user's directory will be affected (of course this is not as good for personal use where user data is more valuable than the OS but it holds in corporate environments)
Plus of course there is security through obscurity, non-windows OS don't have enough market share to make viruses worthwhile. Note that malware does exist on other platforms - take the recent rouge Android apps
edit: love the tails plushy sonic
