All about that money, plus Disney has invested heavily into Epic Games so getting their licenses will be much easier. I imagine once you get one large license, say DC Comics, it is probably harder to get something like Marvel since they don't really let their characters mix all that much in any other medium.
It is a little disheartening to get eliminated by Ned Flanders using a shotgun though, or being chased by that Adventure Time guy half way across the map - creating a bit of horror in the match.
From my experience working with companies since 2009, it is a matter of money and control limitations. For the most part, length of the contracted use and what type of use. Companies such as Nintendo and Sony have done similar things with their Smash Bros games, or playalikes, getting licensed characters from tons of outside companies.
I wish they would do it more, honestly.
I would love to be able to play Super Mario Kart and have the option to purchase non-Nintendo characters to play with. Instead, every company out there wants to attempt making their own kart racing game to compete, then fail horribly, and write it off as a bad idea all together.
There were negotiations to get Nintendo characters for Fortnite but the hold up was apparently Nintendo wanted them to be exclusive to their platforms - I guess everyone else would see a generic Fortnite character instead? Not ideal but I could see that working quite well, actually.
Would Epic go that route? Considering they did not already means probably not. It might be too big a strain on the system. I could see something like that being an option with only others on that same console to play against though - maybe 30 person maps rather than 100 or so.
I know they are doing an upcoming Harry Potter collaboration as well as a few others mixed in that season so that will be interesting. We already had broomsticks for mobility in Chapter 6, I believe.
The only missed opportunity I saw so far with their collabs was Transformers. What do you mean I can get an Optimus Prime skin but then cannot transform and become the vehicle mode for mobility? Plus they didn't bring in many Transformers, I believe only Optimus was used.
Using Megatron, G1 version, would have been awesome for duos - transform to his pistol mode and become the weapon for your partner. Same for his later altmodes - let your partner drive him in tank mode but you control the turret, or vice versa. Missed opportunity.