That may be true, but how many shops do you know of that actually use HyperV? VMware dominates, Xen a ways behind, and Linux KVM and VirtualBox back aways. I don't think anyone actually runs VMs under Windows, it's rather the other way around.
Microsoft has been making some inroads with Hyper-V with mid-size businesses that are already 100% Windows environments - especially ones that haven't quite started down the virtualization path. Their licensing is attractive to these smaller companies, compared to VMware (at least the higher-end vSphere offerings). And it's Microsoft, which they're already comfortable with.
VMware destroys Hyper-V in just about every possible way at the enterprise level, but mid-size companies often don't need all the bells and whistles that vSphere offers, even as cool as they are.