Becasue they work well?
I have an MCSE and my primary home computer is a Mac. IMHO it simply works better. I've worked in a multi-platform environment for more than 20 years. Macs normally generate far less heartburn than Windows machines, particularly for professionals, researchers and scientists.
If Windows 8 lays an egg you may have lots of Macs on your hands. I have a small crew of college interns and they are having fits trying to figure out the preview edition.