Ike Perlmutter..
He doesn't want to do female hero merchandise.
And he finalized the Infinity releases.
Wanda is the most requested and now MCU and its merch isn't under his umbrella which is why on the theater promos and posters Wanda is there instead of Sharon who is on the earlier promo stuff.
It seems to be representative of an old boy's club mentality when it comes to these things. It's not just Marvel though. There was that news of a supposed merchandising insider who said a lot of the Rey merch for Force Awakens was axed so they could focus on Kylo Ren who they apparently thought would be the breakout character. Star Wars and Marvel toys are still almost exclusively boy focused even though there are all kinds of young girls (and adult women) who love the properties. It's just sad and Marvel isn't the only one to blame for shortcomings like this in Infinity.