MvC:I has a smaller cast, on a more powerful console with an experienced staff that worked on those previous MvC games. MvC2 had the largest cast possible and was clearly trying to be an 'all star game' where they crammed as much as possible with the resources they had because they couldn't create a whole roster full of characters.
They still made Son Son, Cable, Amingo, Jill Valentine, Hayato Kanzaki, Ruby Heart, Marrow, Tronn Bone, and Servbots and Abyss as new sprites. That's 10 new characters they created from scratch. Those 10 characters were created despite Capcom could have chosen to find 10 more characters with premade sprites to put in the game. They instead put in the development time and money to do so.
MvC:I also ported most of their cast from UMvC3, and still has a smaller cast. The only new characters of the initial cast are Captain Marvel, Gamora, Ultron, Jedah, and X. That's 6.
Now, DLC will bring all new characters in Black Panther, Black Widow, Winter Soldier, Sigma, and Monster Hunter, and Venom, but that's still just 12 and potentially a year after release.
It's a different service model, but even then, Capcom chose to go with a smaller cast and less new characters for MvC:I.