Microsoft's vendor lock-in is still very dominant, and a lot of sysadmins don't really have a deep understanding of anything else. Facebook's social network effects are analogous to Microsoft's vendor lock-in, but they aren't as deeply entrenched.
Also, mosts of microsofts enterprise products are of quite good quality. especially active directory.
Not to mention the insane, massive momentum microsoft has in terms of MSP's/Partners..