In simple terms, Carol is a lead character who gets her own movies, Wanda is a supporting character who backs up the (all but one) male lead characters. Maybe Wanda gets her own character arc time permits. That’s why Carol gets “a movie to convince everyone she’s awesome”, like Iron Man, Captain America, etc. (Hell, arguably Wanda does too, its called Age of Ultron).Better introduction and superior buildup over time. In simple terms, they didn't create a movie to convince everyone she's awesome, they just made her awesome and let everything else follow naturally from there.
It helps that, for all that she's one of the most powerful people in the setting, she also has fairly clear limits and weaknesses that allow you to write plots which don't break from her presence. Wanda might well be able to take on Thanos solo, but she's a glass cannon who can be beaten, or at least distracted, if enough crap comes in from too many different directions.
Actually, come to think of it, I’m pretty sure people would still be complaining if Carol came in and beat the **** out of all the Avengers before turning good as Wanda does.
Number of movies in service has nothing to do with it. Vision is a literal Deus Ex Machina in AoU and people have less problem with him than the lady who’s a flying brick.