The Crown isn't as powerful as you (and the Crown themselves at this point) think they are tbh, they cannot replace stewards. Stewards are chosen by the citizens of the city they belong to, not the Crown--if a Crown were to try to replace a steward it would be seen as an act of tyranny. The Crown can't touch Steward Welat and he knows it.
Also, a lot of what the Crown can actually do depends on the willingness of the Imperial Court and other high-ranking officials to obey them - such as Steward Welat, in other words. Underestimate him at your own peril.