Doesn't have to be that way.
The reason why RunUO quickly became such a success, and the reason why the rest of the community is pretty much in shambles, is because RunUO got the backing it needed to realize it's potential. Coders, Scripters, etc. gathered around a single UO Emu (yes, it is, no matter what they claim. If RunUO isn't an "UO Emulator" then the rest aren't either.). Sure, there were some pretty damn good brick-layers at the bottom, but we're not talking about 2 guys doing all the work here. Community effort is the keyword.
Then look at the rest of the "scene". So quiet you barely dare speak in case you're arrested for disturbing the peace. Small groups of hopeful developers, all doing "their thing". But without the extra resources (more developers, more scripters, more users - in that order), they'll never grow beyond "small hobby project supposed to offer something 'different'".
Everyone breaks off to do their own version, following their own visions. Which is fine, and commendable. Only problem is 9/10 never get out of the starting blocks, but remains there always having that "potential" hanging over them. I've lost count of how many "Project is on the backburner right now, a lot on my hands, etc. Don't worry, it's not dead." messages I've seen. Famous last words.
Nice summary of the UO Emu scene btw. Though you forgot Sphere in the "Now" section.