Why do you think Poison-type Pokemon were so commonplace and widespread in Gen I and since then have been relatively scarce since?

Honestly, maybe the fact that it’s true is the reason for the thing itself? Like, if balance of the number of Pokémon in each type is something that Game Freak cares about at all, then you could fairly look at the 33 first-generation Poison-types and say “okay, we have more than enough of these.” Per Bulbapedia, Poison is still the 8th most common type out of 18, despite gaining only three new members in generation II, four in III and just two (Skrelp and Dragalge) in VI.

I think Poison is just… a weird thing to even be a type, frankly. It’s like Flying, in that it’s more something a Pokémon does than something a Pokémon is (except arguably in the case of industrial waste Pokémon like Muk and Weezing), and it’s not hard to imagine its abilities being given fairly freely to Pokémon who aren’t actually members of the type. And… well, think of other JRPGs. Poison is always a status effect; off the top of my head I can’t think of any games that have a concept of status effects where poison isn’t one of them. However, I think I’m justified in saying that it’s very rarely, if ever, a trait of monsters that affects their general strengths and weaknesses. Having Poison as a type at all is a very weird decision, both conceptually and in terms of mechanical game design, and generation I also slaps it on several Pokémon for whom poison is… arguably not a very strong part of their identity – Bulbasaur, Golbat, Nidoran? I’m really going out on a limb here, but it’s sort of plausible to me that Game Freak’s designers genuinely didn’t know what to do with the Poison type for quite a while after the first games.