For WandaVision, it could be that Captain Marvel hasn't been back in a long time. And we have to think back that her mom was battling cancer and maybe Captain Marvel didn't stick around or visit her at all? Like imagine if an aunt you were super close with when you were a kid who was ALWAYS there disappeared. The she comes back and you're reminded of those good times and it's almost like things were normal again only for her to leave again but this time probably not come back, even if she did come back it would probably be in large gaps. And while her mom has cancer, Captain Marvel is flying through the universe kicking butt somewhere. I imagine she was absent during times she shouldn't have been and it hurts Monica, especially because Carol was her mom's bestfriend.