Most of the Sylux support was based on two things.
1. People were desperate for Metroid representation and thought Ridey was too big and that Dark Samus would never appear again.
2. The promise of SOMETHING happening with him. A promise that hasn't gotten anything more than ANOTHER teaser since 2007.
Now that Metroid has two new characters in Ridley and Dark Samus, and the fact that Metroid is showing new exciting characters that actually have plot importance, why would one support Sylux unless you were a Sylux super fan?
Sylux may have a surge of popularity after Metroid Prime 4 but... that's assuming he's even in the damn game.
Yeah, this was the big problem with Sylux. With characters people support you usually want something tangible to latch on to, a history with the franchise, a relationship between important characters, fun boss fights, and so on, something that makes you invested in the character and want to see them in Smash. That's why Ridley and Dark Samus were popular wants for Metroid, they were characters we were invested in and loved seeing in the Metroid franchise, so of course we would be invested in seeing them in Smash as well.
Sylux hasn't done that yet. They do have Hunters, but they were one character among many and not particularly important in that game over the others either, so it was not a major selling point on its own. Then we got the promises and teasers rolling in, which of course were indicating they'd be important, but nothing has come of them and has left the character in a limbo for over a decade. That's not a lot to work with especially in the face of mainstays like the aforementioned Ridley and Dark Samus, and now that both of those characters are in the selling point of being the 'only' Metroid option has gone poof. Their flaws are really showing, and as a result many have moved on to other characters that have more immediately interesting prospects like the Rogue Chozo General.
If Sylux is to be a strong contender again, they need to get their selling point out with their next game. Teases and promises can only go so far to sell the character anymore, especially if characters like the Rogue Chozo General maintain their importance and presence from Dread onwards.