Plain and simple, VMWare is pricey. I'd love to run them where I work, but it's extraordinarily expensive compared to Xen and Hyper-V.
Hyper-V is about 5 years behind and XenServer is about 3 years behind in terms of functionality and stability, mainly due to the fact that VMWare has been doing it for so long. VMWare is rock-solid and feature rich, and I'd love to use them. Currently we use XenServer, but with Citrix recently closing down their hardware API's and not playing nicely with anyone it looks like it is going to be the first casualty. I've been very upset by XenServer's HA so far, plain and simple it has sucked. I've had hosts reboot from crashes and the virtual machines go down, but the host thinks it has the machines and all of the other hosts think it has the machines. I've done everything XenServer has asked (HA quorum on a separate LUN, patches, etc), but it still just sucks. I've yet to see a host fail and the machines to go elsewhere, and the configuration is absolutely right and has been reviewed by Citrix. Maybe 6.0 will be better, but I just heard of major issues today with it. Hyper-V is really where the competition is going to come from, especially with how engrained it is in everything coming up. Want to run Exchange 2010 SP2? Recommendation is Hyper-V virtual machines.
God I miss VMWare.