I think the difference is that Captain Marvel HAS a financially successful movie under her belt, so they know she can bring them money when they use the ''right'' approach. Quantumania was Kang's first chance to make an impression, so that coupled with the actor's real life issues probably felt like a bad omen to the studio. Also, he's just a villain, while she is the headlining hero of a franchise. It's easier and there's less to lose by writing him out compared to her.
I don't like this decision either, btw. I believe that Kang deserved better, but the fact that there are discussions for his role to be just minimized at least seems to indicate he can still have some role, which is better than the alternative of just being scrapped like he didn't exist. I hope they make the best out of what's left of his role. If he's going out, at least let him go out with a bang.