Well, there is a couple arguments I can think of that do make sense.
One: first come, first serve. It's not secret that no matter what, not every viable character that wasn't a newcomer before is guaranteed to get in one Smash game's lifespan (until further notice), so quite a bit will have to be possibly left with minor content as compensation, if they get anything at all.
Two: The denial that there will be one more DLC extension that would give Rayman another chance to get in before they close up shop, regardless of whatever possible evidence has been spotted in the code, Ninty websites, or elsewheree.
Other than that, any other argument is nonsense.
It’s basically sensical for Indie or Non-Mainstream Studios, however Ubisoft (and Tecmo) are on the Hot Seat of Fighter Pack(s) because of Nintendo Triple AAA Content Creative Collaboration with Minor DLC Content which is what the Past DLC Collaborators have been as:
- Square Enix creating exclusives for Nintendo Content like Bravely Default, Super Mario RPG, Octopath Traveler, many other games that are/have been on the Nintendo Console Brands. Because of that they have 2-Fighter Packs in Hero
and Sephiroth
Packs
- SEGA creating exclusives for Nintendo Content like Mario & Sonic at the Olympics, Bayonetta, etc. games for the Nintendo Consoles. We than got the Joker
Pack for the Game
- Microsoft owned Studios Rareware and Mojang created Nintendo Console Branded Games like Banjo-Kazooie, Diddy Kong Racing, Blast Corps, Jet Force Gemini, Goldeneye 007 and Minecraft for Wii U. So than we got Banjo and Kazooie
and Minecraft
Packs out of it.
- SNK and Sakurai have been the sole purpose of the existence of Super Smash Bros entire essence and creation, which we would get an entire SNK Collection of Content in the Terry
Pack.
So with that being said Ubisoft (and Tecmo) are integral in Nintendo’s success with Exclusive Nintendo Products that Nintendo will Without Question go to for Fighter Pack content, as you mentioned it’s “First Come, First Serve” but only to Nintendo Selected Collaborators that helped put their own Brand in the Success they are in right now. Especially when it’s Collaborators that have Switch Content they no doubt want to promote like Rayman Legends internationally including Japan (and even the Ninja Gaiden Collection coming to the Switch)