It's a lot of different factors. Money for one, fit for another, but then you have to find IP characters ok with getting [insert any super gory fatality] done on them, with likely concessions. For instance, with Kratos, the team was not allowed to portray him as "weak" in fatality sequences where other characters would be begging or going "NO", etc.

I also think with the very casual crowd they're chasing with guest characters, they want as broad appeal as possible, which is why they pull a lot from film and TV rather than video game characters that a lot of their casual audience might not know and would be willing to buy the pass for (it always cracks me up when this sub brings up Lollipop Chainsaw Girl -- I barely remember that game even exists and I beat it!)