It just doesn't make sense for most modes to stick around permanently. Practically speaking, that means they have to maintain multiple modes instead of just updating a mode to suit current League periodically. Even if ARURF is super low maintenance, its still requires updates every time items, Champions, SR, SR jungle, etc are changed. It pulls resources away from other modes. Keep in mind that, while ARURF is quite popular, there are plenty of people that don't play it/prefer other modes.
That said, I would like to Riot go back to more regular rotating modes again. The problem previously wasn't lack of interest but lack of variety, imo: literally just the same 3 or 4 modes (Ascension, Poro King, Nexus Siege, rinse, repeat) that people got tired of. Like, where is OFA? Why are they limiting it so much? Star Guardian mode? Literally only a handful of Champions and the items have to be accounted for, relatively low maintenance.
I liked how long ARURF stuck around for, too. Basically, while I understand that most modes can't be permanent, I don't see why they can't have modes stay for much longer. Why is it like a week or two and gone most of the time?