The budget of a film like that would be too huge for it to be made up for by the box office. Not to mention the production company making it would have go through a ton of copyright protection from not just Marvel and DC, but also from other production companies producing the superhero films. And the fans of both comic universes wouldn't be satisfied with what the film has to offer since a Marvel vs DC film can have a plot that could go any way.
The movie directors aren't stupid enough to pay that much money to hire that many actors. There would be way too much clutter on screen. Nobody would know what's going on because there's 300 people on screen at once. They already spent so much money on the Avengers, and plus, that's only FOUR Marvel members (The archer guy, that hot chick don't count.) Imagine EVERYONE THERE! How many superheroes are in the Marvel Universe. I DON'T EVEN KNOW! I know there's not FOUR! I know that.