Since you said other games in the genre, I have a theory to why Smash as this sort of conundrum
-Smash has a much bigger install base than other Fighting games. It's easier to get more hardcore speculators from the 20 Million that bought the game than Sub 10 mil that other Fighting games have
-Being a Crossover game, especially nowadays with many characters from other parties, there's many characters and things that could be speculated about when it comes to Smash. With other games like FighterZ, Tekken, Street Fighter, the roster is mostly limited to the characters from that franchise, so it narrows down the options to talk about (Tekken has guest fighters, but they stopped happening after Season 2)
-The Rampant nature of Rumors/Leaks/Theories juices up the discussion in the speculation community, so there's usually something new to talk about, even if fake. Other Fighting games do have this, but they are Nowhere near as frantic as it can be for Smash. There's less of Rumors/Leaks nowdays during this speculation cycle, and notice how when there's less of those, there's less discussion being around Smash
-The Advertisement for Smash is unlike most of what I've seen from other Fighting games. For Base game, There was usually something new to look forward to everyday, (well not counting the Weekend). these could just be new pics of the game or some actual news, but it would keep people on the edge of their seats and such. Other Fighting games usually show something new like every month or 2, and then just disappear.
Along with that, Smash 4 and onwards, the Trailers became a large spectacle you don't usually see from Fighting game trailers. Usually Fighting game trailers usually only use Ingame footage, they showcase the moveset, the render, and that's that.
Smash uses these high quality produced trailers, usually involving CGI, to show off their characters. It's just something you usually don't see often, and combine this with the fact that the So many options for what the trailer could be for, considering it is a Crossover fighter, and with each reveal, people are most especially gonna be paying attention and are going to be on a large High from the trailer. It's not just the surprise of who the character is, but also that the surprise itself looks so Epic and awesome and such.
And the final aspect of the advertisement is the presentations we started getting since Smash 4. These presentations, especially during Base game, would show off New info on large parts of the game, and it's exciting to behold. They've also are a large part of the reason that the fanbase has such an attachment to Sakurai, since with these presentations, we get to See Sakurai in his glory (of his public persona at least). Most other fighting games just show off their trailers and that's that. Trailers are cool, but a whole presentation on some of the things coming to the game is tenfold compared to a trailer. Even with the DLC Presentations, there's usually something new to see we didn't see in the trailer, like the Mii Costumes, the amount of music, and other miscellaneous things, beside the moveset
So all of this combines to form a big attachment to the hype cycle of Ultimate for some people, where they can't get enough and Smash consumes a large part of their mind. even if they only play the game for like an hour or less, or with the DLC character for 5 minutes, the High from the trailer + the presentation is one that people like to chase.
The one fighting game most similar to Smash in terms of speculation is Mortal Kombat. That does have some dedicated speculation community and alot of hte time has had alot or Rumor/Leaks popping up. I will say that in comparison to Smash, some aspects of Mortal Kombat speculation fall short. There's not news everyday like there is with Smash (base game), The trailers are usually just Gameplay footage when it comes to the Characters, I think the Presentations for Smash tend to be more concise and straight to the point compared to NRS, and Sakurai seems to carry himself better as a persona than the NRS Members, There are guest fighters, but I don't think it branches up to the amount of options Smash does, and Mortal Kombat doesn't have as high sales numbers as Smash does, even though it's also sells above the average of other Fighting games, even getting past 10 Million