I'm not
@demetre but from what I've read the most realistic scenario seems to be that Zelenskiy felt threatened by Zaluzhny's political potential. The lack of success of the counter-offensive didn't seem to affect public's perception of the army (and Zaluzhny) as they're rightly seen as heroes who give their all to defend their country. But it did affect Zelenskiy's own rating.
Here's a survey that says that the army's (and, by proxy, Zaluzhny's) approval rating is between 93 to 95 % (depending on the region) and Zelenskiy's is 75-76%, which is still a lot but it's way less than it was at the beginning of the invasion.
Plus, there were always rumours that the two didn't get along very well — Zaluzhny did a lot of things his own way, didn't consult the presidential's office before giving big interviews (like the one for The Economist) etc. As the situation on the front is currently in a relative (bloody and horrible) stalemate, Zelenskiy
probably decided that if there ever was the time for this move, it was now.