I didn't read the above replies, but the main reason is approx. 1.2 million viruses, rootkits,
trojans, spywares and worms affecting windows vs. approx. 25 (4 orders of magnitude less )
affecting OS X (all patched).  how does one put a price on the damage done by windows
malware?  perhaps a few dollars more is worth the security and stability.

there were approx. 5 dozen malwares affecting the now-extinct pre-OS X versions of mac OS,
for any historians here.

regarding the various softwares not ported to OS X, the clever user always finds a way,
hence it's no handicap.

p.s. for the literate here, MAC is an acronym for media access control.