You're seeing the whole thing retroactively, evidenced also by the fact that you are mentioning mons that didn't even exist in XY and so couldn't affect the viability of Slowbro when it rose (as well as unsets like Charizard Y without Solar Beam). When the teleteam was first created Slowbro was considered a pretty niche mon, with the presence of Aegislash putting a seemingly huge burden on its viability. Saying that Tele singlehandedly made it rose is kind of a tongue in cheek meme, but it is true that he was one of the earliest players popularizing Slowbro and obtaining consistent results with it.Slowbro was a really splashable Pokémon that could act as a glue mon on a lot of teams, checking stuff like Keldeo, some Mega Medicham sets, Talonflame, Alakazam, Lando-T, non-SolarBeam Zard Y, Mega Lopunny and non-ThunderPunch Megagross. The main reason for its severe drop in viability in USUM is the rise of mons like Koko and Bulu who just shit on it, as the other main Regenerator, Tangrowth, can handle those two much better, and the other main bulky water, Toxapex. The advent of Z-moves also allows some of the things that Slowbro was supposed to check to easily remove it.
Mamo isn't nearly as splashable as Slowbro was (as it gives like no defensive synergy) and faces tons of competition from other Ground-type wallbreakers like Excadrill, Garchomp and offensive LandoT, and Ice-types like Kyurem-B and Weavile.
And I don't know why you keep bringing out Excadrill and Garchomp, one of which is relegated to lead roles, and the other's only semblance of a niche is being a semisuicidal check to Hawlucha, and claim that they are better than a mon that forces out a good half of the metagame.