It could have been ANY pokemon. Ash could've traveled with ANY pokemon. But out of the first 151, it just had to be Pika. Why?
Bulbasaur's pokedex entry is #001, so it's not hard for him to be a candidate. Heck, the three starters all could've had the chance (of course, they all later joined the team, but as supporting characters, NOT the main protagonist)
It could've been Mew as well. I know Mew is a legendary... but it's not really all that strong compared to other legendaries, no?
So out of all those pokemons, why - WHY Pikachu? Any guesses?
EDIT: This is a thread about "Why Pikachu Was Chosen?", not "Why Clefairy Was Not Chosen". Clefairy isn't the subject here. Thank you.