When your average punter thinks Marvel they think Iron Man, Spider-man, Avengers, Thor, Hulk, Guardians etc.
They don't think Alien. Alien is long way down Marvel's priority list. And unless you're an Alien fan or comic geek, it's not even on your priority list.
There's not enough negative press for them to worry about it. If there's a backlash against the Land cover in terms of sales - then they might take notice.