Overrated? If I had to pick a level 1 hypervisor for a production environment id probably choose VMware without hesitation.
If Xen has a show stopping bug I get to patch it myself.
Oh you mean technical reasons? None.
Alternative exist, but VMWare has a lot of mindshare and their products are pretty good, so they're hard to beat.
Didn't Citrix buy xen a while back (or some part of it)? They had something called xenapp, now apparently "Citrix Virtual Apps and Desktops (XenApp & XenDesktop)"?
Not really the same as "a supported xen distro" I guess.
Also, I'm pretty sure if there's a Xen bug you can throw Citrix under the bus (assuming you're paying them for enterprise support).