Honestly, it sucks, but Rayman(not that he's my personal choice) is kind of in the lower of the three core choices of franchises due to having the weakest showing in Japan.
But hey, that could be entirely different, as things like fan demand do matter too. Albeit, part of what helped BK was Microsoft getting in good with Nintendo via Minecraft alone, but Nintendo and Ubisoft were already pals for a long time. So it's at most a matter of how the ballot went. Rayman is the only straight franchise alone among Ubisoft's to get an NPC appearance in base too, which while could be argued as "Nintendo published some games in Japan", it does mean that it's at least considered important. Regular Rabbids didn't, but the crossover did. And AC didn't show up till DLC, same with Rabbids. Despite one having a weaker Japanese showing, it's pretty clear they're all held in high regard anyway, and that's great for all fans as they feel like a pretty good reasonable chance. Even if Rayman isn't playable, as long as Ubisoft is game, I expect some kind of NPC appearance or costume in some way. Same with the rest by now, albeit, some would be DLC too. But Spirits don't have to be, as shown. If you think about it, that's also kind of what Rayman had that regular Rabbids and AC didn't; he was able to be put in base game cause he wasn't a costume. Next game could change this up quite a bit.