All I'll say is that Smash Ultimate's development lines up with the Smash Ballot, and Rayman was most likely a very popular vote for that.
Ah yes the smash ballot, where at best you can make an argument that at best only 4 of our DLC characters came from.
Now let me break this down for you.
Joker: Considering P5 did not launch globally until 2017 and did not launch at all until 2016, I doubt the ballot could have had any effect here.
Hero: Now we could make an argument here, but saying this is due to the ballot also is kind of difficult to do. The limited japanese polls we have at the time do not suggest that Hero was a super popular pick, though this might have been due to the fact there are multiple DQ heroes.
Banjo: The only pick that you could argue came from the Ballot. A super fan favorite since Melee.
Terry: Once again, the data we have is not strong enough to make an argument that Terry came from the ballot. You could say since he is popular in Latin America and Japan that could have helped, but even moreso than Hero the polls dont really show it. While these are admittedly a small sample size for polls, the fact is we do not have any good data here.
Byleth: No, Three Houses did not even exist.
Min Min: See Byleth.
Steve: Maybe you can say there was a silent majority clamoring for Steve, but his support did not start to rise until after Smash 4's ballot was done.
So you have one character that you can make a strong case came from the ballot. You have three that have some evidence you could say did ok on the ballot but I would say each has arguably easier arguments than "lol ballot". And you have three that flat out did not.
See how relying on the ballot does not make sense right now? Don't worry, it is ok that Rayman is not going to happen.
Lol there's no such thing as only having a chance in one specific smash game
Except that there is. It turns out, characters tend to do better when they are in the limelight or cultural zeitgeist.
Even ignoring the low hanging fruit of FE and Pokemon, there are several examples where someone had a much stronger argument for one game than the next. Remember Isaac in Brawl? How about Elma for Smash 4 DLC? Or how about Krystal in Brawl? That is three big "fan favorite" examples of characters that had reasonably strong pushes for their respective timeframes from fans and could have been seen as having strong evidence in their favor. And then look at how the evidence for their inclusion weakened for the next game after that.
Rayman was in the middle of a comeback with a big then Wii U game in legends in Smash 4. He had some degree of fan demand that exploded after the Rayman leak. If there was a time where the stars aligned for Rayman, it is Smash 4. Rayman now is a franchise that is dormant and is no longer the hottest character in speculation due to a high quality leak. So, logically, Rayman had a stronger argument in Smash 4. Therefore, he did miss the boat if his chances seem bleaker now.