I actually forgot about that because it's been so long since I rewatched Kanto. Why did he do that again? I can't remember? Also, did that have something to do with his Ritchie loss?? Did he only have five Pokémon on him at the time, and if he had had 6 he wouldn't have had to use Charizard?
Which is? Even back then, his team decisions made no sense, though at least we can say that his rookie status was to blame.
Forgive me as my memory is a little sketchy, but if I remember rightly... he had Pikachu and obtained Caterpie, which quickly evolved into Metapod and then Butterfree, Pidgeotto, Bulbasaur, Charmander and Squirtle, making up his original six. While he had these six on hand, he caught Krabby, which went straight to Professor Oak. Butterfree was very briefly traded for a Raticate, before trading back, and then departed permanently, being replaced by Primeape, which also departed shortly after, leaving him with five. Obviously he could've "withdrawn" Krabby at this point but he never did, and I can't remember this point ever actually being addressed. He then caught Muk, which briefly took up the sixth spot, but it was also "Oak'd" because it smelt so bad and, somehow, could be smelt from inside the Poke Ball. Ash only kept five Pokemon on hand from here on in, despite Krabby and Muk being sat at Oak's, and his only other captures were the 30 Tauros, which went straight to Oak's.
The reason for this, obviously, falls at the feet of the writers. Personally, I assumed that they were finding it hard enough to equally share out the screen time between Ash's five Pokemon on hand, without adding another. The focus was very much on Pikachu, Bulbasaur, Char-mander-meleon-izard and Squirtle, with Pidgetto more of a backseat. This is just what I assume and is in no way factually accurate, but I struggle to see how they could've included another character on hand for Ash, or created a good character that wasn't comparatively neglected! In-universe, this all means nothing, but I can't recall any reason being given.
Again, with Ash's battle with Ritchie, I can't really remember the issue of Ash not having many Pokemon being bought up specifically. All I can remember is Ash saying that Pidgeotto was exhausted having flown a Team Rocket balloon back to Indigo Plateau.