I never realized that...interesting. I think I'm going to go find out why. I could see some of our smaller customers using Essentials Plus wanting to do it...especially now with HBR and the cheaper <75 VM licensing.
I just assumed that it would run on all editions of VMware as well. I was thinking of getting two essentials plus licenses for both sites (since you need VCenter licensed in both locations). Now the cost doubles with learning thsi new piece of information.