The thing with Skull Kid is that hes popular. Not necessarily smash community popular, not necessarily ballot popular either, he is just popular. Go into any store and look at the zelda merchandise and i guarantee there is majoras mask stuff on the t-shirts, wallets, pins, hats etc... that mask is iconic to zelda just like the triforce and master sword.
The thing with skull kid "suddenly" becoming a likely character isn't so much people don't want him as it is circumstances in the past made him seem less likely. sakurai adding any "non-triforce holder" zelda character has over the years seemed unlikely as he's skipped adding midna/ghirahim/impa/tingle etc.. in the past and in the smash community guessing at those characters became more and more "unlikely" feeling based on sakurai adding multiple links and multiple zeldas instead (even toon zelda ALMOST happened in brawl).
In smash 4 his assist trophy was revealed early on so no one guessed at him much, there simply wasnt time to build up a sizeable fan base during that speculation season. For the ballot he was already an assist trophy so people looked over characters like him or Ridley and assumed they might be "out of the running" and decided not to vote for them, as it might be throwing the vote away. most seemed to believe the ballot was for smash4 DLC and the side note about "future smash games" I dont think swayed many people to vote for those "deconfirmed" characters much. of course we GOT ridley so sakurai is aware of non-ballot based popularity, and thats really where i think skull kid shines and makes sense, as i said before he's a very popular and well known nintendo icon for the zelda series at this point, regardless of ballot votes.
Because of this history (non-triforce holders seeming unlikely zelda choices over the years, skull kid being deconfirmed early on as an assist trophy for smash4, likely not getting a lot of ballot votes due to those first two issues) Skull Kid hasn't been talked about much in the lead up to Ultimate. Then we got the newest direct and with the moon becoming an assist trophy without skull kid (who could easily be part of that, calling the moon down) and loz18 "leaking" skull kid (however you feel about loz18 he did have correct E3 info and it was mostly zelda based info specifically) now skull kid seems very likely. theres also some wild tinfoil hat theory stuff out there stuff like the sakurai office backgrounds etc... but i think people really do want skull kid (i know i think hes cool) and i think he's a popular character for the general nintendo fan base. He only got looked over so much in the smash community due to a series of circumstances and not because the smash fanbase doesnt desire the character be added to the roster, now that he seems possible and likely the fan support is simply being highlighted in a way it never had a chance to be before.