Most pedos/troons/gays that pretend to have an interest in pokemon for.... less than squeaky clean purposes tend to just be following a fad (gays and troons especially).
For that reason that don't really give a shit about pokemon, but desperately want to appear like they do. Bulbasaur is a starter and literally the "first" pokemon in the dex. Its an easy choice and because its so old and popular its easy to amass cheap merch and fanart to seem like a real fan of the mon. If you're a pedo thats a plus because kids would be easy to fool/lure with such items.
Also people latch onto sylveon for similar reasons, except for whatever reason people think that particular fictional animal is mentally ill just like them.
