These are just different projects type. With FF and Avengers he had the reins of the franchising, i.e. he was writing basically all the books. X-Men always had a lot of titles, and knowing he could only write a finite amount it didn’t make sense to make something completely predefined and force anyone else to write his desiderata, as a creator he probably has more respect for his peers that most readers have, and as an adult person he probably doesn’t have a “I want everything done my way” aptitude.
He said since the beginning that this project was different and he didn’t made a issue by issue roadmap but just defined 3 phases that could go on indefinitely if readers liked them. Probably they didn’t expect the Krakoa phase to be so successful with readers and this made plan change a bit; we clearly entered part of the second phase, the expansion in space that Powers of X talked about, but at the same time we didn’t leave the Krakoa phase behind yet, Inferno will change things for sure but I don’t think it will be the end of Krakoa, unless they want to shut down all the books based on the premise (basically all the books except S.W.O.R.D. and partially the new X-Men title). Moreover the Orchis, Hordeculture, Children of the Vault and several other plots have not been solved yet.
What it could be is that between writing a 20 pages book monthly + specials or a couple of 160 pages maxi series like Inferno per year, where creative and artistic integrity are easier to maintain (no fill-ins, double shipping and so on) he preferred the latter approach, who wouldn’t. Perhaps he will continue to coordinate things and co-write with other creatives here and here to make sure the things he needs are into place for his maxi series with top artists. Or maybe he is going to write a monthly more in his wheelhouse about Moira, the Council etc. like he did New Avengers while leaving the classic action-driven heroics to other writers like Duggan, he doesn’t seem to care a lot about them anyway.
If he is leaving is not for some power struggle that would make no sense at all (all books have been free to go in thei direction they wanted as much as they wanted, and the need to respect some broader story guidelines is not something specific to Hickman’s X-Men, all mainstream comics work this way, especially when one writes satellite books) but because he may have got a better gig, like TV or movies.