Well yeah, UC has the most staying power... because it's the only one they built upon to that extent.From my understanding, that isn't entirely the case. The biggest problem is that no other continuity has the sheer staying power that is UC Gundam's thing
Having a back to back stretch of three shows, plus the original and its trilogy adaptations, plus CCA, plus 2 OVAs, all before the turn of the century gives UC a critical mass none of the other universes has.
SEED's the only one that comes close, with two shows, but that's when they discovered their fojoushi demo doesn't buy the Gunpla, they buy all the other shit. Then Morosawa's brain cancer really got in the way.
The problem with Gundam is that they keep trying to funnel everyone into UC, instead of just accepting they have a really big fanbase that likes different things - especially the western fans, and keeping up a steady supply of OVAs for the non-UC folks. The only move they've kind of made towards this is the Build stuff, which is less serious "wow, cool robots" material, but even then, they fucked it up with the unnecessary Fighters/Divers continuity split.