Eh, I don't know if I'd say it's "super-popular IPs"? Batman is big, but him being a character in another fighter doesn't make it a blockbuster. After that you have GoT characters and Harley and I guess Shaggy is trendy weirdly? The Adventure Time and Steven Universe characters are big with gamers in a perfect demo but neither has sold a game by themselves to a great degree. And then you get into Tom and Jerry, or Bugs Bunny, or Wonder Woman, or other characters from the 1940s...
I think it's partly the meme culture and then it's the aggressive Twitch promotion of this (WB apparently went after streamers, knowing that both sides love giveaways) to hook players with a clever version of the F2P model where initial joiners have the idea that they're still getting something of exclusive value even though it'll ultimately be free. Plus the character unlock system keeps players persistent longer, hooking them to play an extended amount to get all the initial stuff.
On paper, MultiVersus looks like it's got little chance of standing out as more than just another forgettable Smash Clone for kids, but there's something to the rollout that is working.