Because I only give a shit about a few characters and don't want to spend money for the rest? I'll probably never play as them. I'm sure I'm not the only one, either.
I realize it's the same with the main game's character roster too. I paid full price for the main game, even though I never play as She-Hulk or MODOK or Spencer. Didn't have much choice there, and it's disappointing that I'm still not given any choice about the expanded version either.
Just my gripe.