This is complete speculation, obviously, but Sega probably doesn't want to have to divert any more resources than needed to PSO2 when they can instead be focusing them on NGS. It's why the festivals are on a set routine now yearly going forward in PSO2, and it's why there's no more mission passes, or anything that would require the development team to manually update the game every few weeks to make adjustments to them.
Now, granted, Sega could do something like make a "Permanent" schedule of UQ's for PSO2, but we don't know if that's feasible to do on their end, or if it would still require them to take manpower and resources away from NGS to do so. Their aim seems to be to put PSO2 into a state of permanent automation, much like the original Guild Wars is right now, so that outside of needing to fix something if it breaks, they don't need to actually make any adjustments to the game during weekly server maintenance (which, given that NGS and PSO2 both exist on the same server mainframe, doesn't require them to specifically target PSO2 to maintain).
Basically - having a rotating UQ schedule would still require direct maintenance by the development team that Sega wants to shy away from, at least for now, to focus entirely on NGS. Obviously that might change, NGS might totally flop, and Sega might double back to PSO2 and instead create Episode 7 or something. I don't know, but right now, it's very clear that Sega's goal is to make PSO2 autonomous, and make NGS the focus for all their development and resources.