Think it's mainly "in group / out group" psychology coupled with how the fanbase was shaped. The casual / competitive conflict in the early days (2003-2004) shaped many of the fanbase's viewpoints: for example, the emphasis that the Melee community should be tight-knit, effectively creating a sense of family, etc. In a sense, the Melee playerbase quickly learned to roll with the punches, and that lead them to embrace the game. Most flaws the engine had was luckily enough not entirely game-breaking, and those that did (wobbling, edge-stalling etc.) were if not hotly debated outright banned quite quickly. For example, one of the more disliked mechanics even amongst Melee players is the edge granting full invincibility on regrab. Stalling the edge was quickly banned as a result.
Also, the Melee fanbase is heavily built around its own stories and legends. I mean, we're talking about the fanbase that annoited its foremost players "King of Smash", "The Five Gods" and "The God Slayer". Those are not small titles.
There are certainly a lot of quirks Brawl / Smash 4 have that are charming in their own right - DACUS, for instance - but in general the Ultimate playerbase (and Brawl / 4, which is where most of Ultimate's competitive playerbase comes from) do not share this sense of "family" and the overly protective streak as much.
Also, some of the main (un)intentional quirks Brawl / Smash 4 had did not have the same capacity for expanding depth; or those cast a major shadow over the games. Hitstun cancelling in Brawl effectively created an extremely defensive metagame, which did not pair nicely with tripping, chaingrabs still being a thing if not even more dangerous than Melee's, or the edge still granting full invincibility on regrab. Characters like
and
were extremely well-positioned to take severe advantage of that: to the point where tournament organizers had to specifically invent or tighten rules just to counter Meta Knight. And that was with edge stalling already being banned. Not to mention
the Meta Knight ban which fizzled out when MK players threatened to no-show. The ban's failure severely undermined Brawl's competitive scene as a result and left a sour aftertaste.
4 meanwhile had major issues with super-early KOs throughout its life, something that was cast-wide thanks to Rage and a lot of kill confirms or 50 / 50s off a single punch or grab (and those problems were exasperated by the extremely high set knockback**) but
and
were released primed and ready to take full advantage of that. Bayo alone caused a major decline in viewership and enthusiasm, with EVO 2018's Grand Finals effectively being the last big nail in the coffin. 4 was relatively well-balanced with most of the cast (something Ultimate heavily improved on), but had significant problems throughout its life with the most dominant characters often being downright opressive.
Those problems cast a major shadow over Brawl's and 4's legacies to a degree the Melee's - or hell, 64's and Ultimate's - communities have not been forced to deal with (outside of the allegations a couple summers back, but that's a still very thorny topic). While there is a bit of a double standard, I point these out since they have shaped the discourse a lot.
Some did, but a good chunk went on to form the core of 4's and Ultimate's playerbases. Players that started out / rose to relative fame as Brawl players include:
- ESAM (of "PIKACHU BUSTED" fame.)
- Dabuz (probably the most well known to this day, has complimented his Olimar with Rosalina and Min Min.)
- Nairo (no need to introduce him.)
- Glutonny ( loyalist and the most well known French player.)
- Kameme (one of Japan's stalwart players, began as a Wario player until he swapped to .)
And so on. A couple of Brawl's Japanese players went on to become playtesters for Ultimate. Most notably 9B, the guy who developed
's infamous chainthrowing shenaningas. My headcanon is that Sakurai contacted him just to make sure that didn't happen in Ultimate.