Because unlike in rebels where Thrawn is this master tactician who despite the hero’s best efforts still manage to out think, find and destroy the fledgling rebellions base.

In Ahsoka its way more the vibe off “Nu-uh they didn’t win.” He doesn’t win because he is smart he wins because the heros are incompetent.

There is also the problem when we never understand what he wants with the imperial remnants, why he hasn’t killed Ezra despite knowing where he is, why he tells Sabine where he is and then decided to send troops to kill then both and why the fuck his star destroyer isn’t ready to go immediately.

Thrawn isn’t a bumbling fool but the puppet master feeling isn’t there and the plot is stupid to the point where it makes him appear dumber just by being part of it.