Well Oak actually didnt have any intention to give it to ash at first because he was doing some research on how to control its power or something he even hid it under the stand where charmander squirtle and bulbasaur were and sense ash really wanted to become a Pokemon trainer and he was a known of proffesor oak he decided to give it to him.
NOTE: I am not really sure about this answer correct but you can always look in youtube for the Pokemon first episode there must a bunch of videos
