The first gym is rock, the second is water, the third is electric. Charmander has a really hard time in those first two matchups and only hits neutral in the third. At this stage of the game, it’s really difficult to fill out your types because the wild Pokémon are largely birds, bugs, and rats.
If you’re really lucky, you’ll hit the extremely low encounter rate for Pikachu in Viridian Forest, and if you’re even luckier, you’ll catch one. But that only helps you against Misty.
In contrast, Bulbasaur is super effective against the first two gyms, and is merely neutral against Lt. Surge and his electric mice.
And by that point you’ve fed two whole gyms full of trainers and gym leaders into your Bulbasaur (by virtue of not having to constantly rotate through your back line just to survive).
Which means it can both take a beating and lay a smackdown in what is on its face a “neutral matchup”.
After that point the game opens up dramatically and that initial advantage fades.
Coelacanth@feddit.nu 6 days ago
The first couple of gyms in Gen 1 are actually really good for Bulbasaur, and getting a strong early game is typically what you want from a starter as mid to late game you’ll have access to a bunch of other 'mons to cover what types you need.