This is because although viruses and other "classic" types of malware still exist and are threats, you're more likely to be hit with a social engineering attacks such as phishing which depend on tricking users into giving up personal information, visiting a poisonous Web
site, or clicking a bad link.
If you don't fall for the traps, you'll significantly reduce your chances of a malicious attack. Additionally, many attacks come through vulnerabilities in Web browsers and software, which has nothing significant to do with which operating system they're running on.