Supposedly due to the schedule conflicts, since he is the one with the career outside the show. I mean, Middleston has fucking Verizon commercials and the indian prick is going to shit out the next Appatow virtue signaling flop comedy with that crowd, but have any of the other main guys in this show been doing anything outside of it? And how the fuck is Jain Yang going to play off of without Ulrich? Without tubby stoner to justify his behavior, he is just another asshole passive aggressive Asian in the bay area (one of a million).
Of course if this is all because Miller was stoned all the time and not meeting his commitments, then yeah fuck him. But I kind of doubt that was much of a factor with the numerous commitments the guy has outside of the show. And it sure seems like Mike Judge (the real genius for keeping these drama whore stoner millennials going) was firmly interested in more stuff with Miller, rather than less. I am guessing Middleston and Berg were being self important twats and Miller was dumb enough to call them out on it in the open, which led us to here. As useless as Ulrich might have seemed to the show, he was the everyman con artist that the others played off of a lot. The only guy who could step up into that role, without adding a whole new cast member, would the "This guy Fucks" dude which might not be bad actually. If you take Miller at his word (stoners are often to high to lie well), I think Judge knows a lot of this and is probably scratching his head right now trying to solve those issues and account for the lack of a charismatic asshole to rope in the non-tech viewers.