My guess would be support. I go to a lot of virtualization user group meetings and have never ran across anyone who uses it. That of course don't mean it's a bad product. The online support for Vmware and Hyper-V is very large. It's easy to use, rock solid and has enough functionality in the free version you may not need to buy anything.
If you're really an OSx fan then Xenserver would be more to your liking. Lots of functions can only be done from command line. I personally don't like it. Our Citrix Xendesktop environments run on it but I'm in the process of getting it migrated to a Hyper-V environment.