>>53050652lack of nostalgia factor and recognizable designs alongside generally poor quality and little marketing for most
none of them have the benefit of being full-on fads at any point save for Yo-kai Watch, and that was only in Japan. Their actual designs are almost always incredibly sub-par animal + elements that just get bigger (name a single Nexomon, Coromon, or whatever-else mon that doesn't fit this mold, I fucking dare you), and since the vast majority have gameplay that's near-identical to Pokémon (the newer games without side activities, too) it makes sure they're never set apart from Pokémon in the brains of people who play them.
The few lasting mon-catching franchises that don't follow this- SMT, DQM, YKW and Digimon- all have:
>drastically different gameplay (basically everything save for arguably YKW)>cohesive design styles with fundamentally different monsters (DQM focuses on Dragon Quest enemies iirc, SMT is a lot more literal and deals with interpretations of actual myths instead of monsters based on them, Digimon makes everything edgy enough to stand out despite having relatively similar ideas behind its designs (in regards to being similar to Pokemon), YKW reinterprets actual folklore *alongside* original creatures that fit a similar niche)and don't even bother trying to go for the Pokémon audience (as opposed to a lot of new competitors in the market who often market themselves very similarly to new pokemon generations, via showing off the obligatory "starters" or a top-down region)
there's also the obvious fact that a shit ton of these "killers" are either made by smaller studios or outright indie devs, instead of a studio like GF that, while still small, at least has the endless Scrooge vaults of Nintendo to back it up with a shitload of marketing and funds
you could also argue that a lack of promotion through mixed media plays a part, especially since Pokemania heavily relied on the anime and TCG when it was a thing, but eh
