Marvel Avengers flopped, and I believe the main reason is the obvious one: The gameplay was not good enough to hold the playerbase. Few missions with few enemies on top of shallow combat can only hold you for so long.

However, a minor, but important factor is the characters. Instead of picking the popular movie version of the Avengers they picked some slightly alternative versions, probably to pay less licensing fees. However, people don't want Chinese rip-offs, they want the real deal.

And of course, there is Miss Marvel. Let me open saying I don't like Miss Marvel, mostly because I don't like any of the new Marvel shit but also I'm old and cranky and I like my classics. I also don't think she fits the Avengers, mostly because those are veterans champions who know their stuff and she is more like a teen hero doing teen antics. 

She also represents the "new" Marvel which I really don't like. Or like even less than when I stopped reading. I mean, at this point they can do whatever they want but if they want me to buy their game it should try to appeal to me and if they don't they should hope their new public want to pay for it.

But story and characters are minor aspects. The meat-and-potatoes of this failure is gameplay, or lack of it. Perhaps they should spend less money on licensing and more in extra stages and enemies?