Besides being popular, recurring, and marketed, Garchomp is also fairly important in-game (relative to most mons at least), being the signature PKMN of one of the most iconic Champions (which makes it a "final boss" of sorts, or at least gives it an explicit role in the game more than just being a random encounter). Maybe you might consider that a minor point, but it's something.
Gardevoir was almost surely explicitly chosen to be Diantha's main in XY to simultaneously market the Fairy-type and Mega Evolutions, which adds another fairly prominent appearance to being Wally's main in RSE.
You can look at mons like Dragonite, Metagross, or Haxorus as also benefitting from this (again, for however much it is worth).
The bigger thing anyway is that certain mons just get clear marketing and continued relevance favoritism, which makes them better candidates for Smash.