It also doesn't help that Ubisoft doesn't do much with the franchise nowadays aside from the occasional port of Legends. I feel like a lot of the Rayman hype from the Smash 4 era came from the positive reception of Origins and Legends, so it makes sense that the hype would die down when Ubisoft just sort of forgot about the series. I feel like Rayman would still be a big name in speculation if Ubisoft continued to capitalize on the series' renaissance, but they let it fall back into obscurity so people stopped caring as a result. Hell, I saw more people discussing the possibility of Rabbids during Ultimate's hype cycle than I did Rayman, which should tell you how much the series' dormancy hurt the character's hype.
I feel as if one of the reasons why is simply one question:
"Where do we go after Legends?"
One of the main reasons why AC's been (almost) yearly is not just greed, but also the very wide variety of possible settings (and times) to choose from. I mean, III and Black Flag are not far from each other time wise - whether by release or by setting - but both games introduced new stuff or expanded on things considerably. Ditto Origins and Odyssey. I mean, they've gone 8 main games and never touched Japan as a setting.
Rayman on the other hand is not really as clear. It feels like they did everything they could do with Origins and Legends - not just in terms of "how much stuff can we get into the games?" but also in terms of effort. It's not every time you get to have a musical autoscrolling platformer with Black Betty as the background song. But where does one go with the 2-D New Super Mario Bros-esque games?
Going back to 3-D might be semi-tempting, but I'd argue that Ubisoft got a wee bit scared after Sonic had a very wonky stretch, the first Yooka-Laylee didn't do as well as expected (not commenting on the game's quality, just expectations) and Rayman 3 being the last 3-D title, mixed bag as it was. They'd also need a new dev team able and willing to tackle a 3-D platformer. Not impossible, but I get the feeling Ubi would've been more confident about a 3D Rayman had Sonic Forces or Yooka-Laylee been successful.