He left his Charizard in a preserve of Charizards (all of whom were bigger and stronger than Ash's charizard), so that it could get stronger, and maybe get some love from the lady Charizard.
Ash usually leaves his Pokemon for two reasons: They can get stronger from someone who is a trainer of that specific pokemon (Charizard and Primape fit this) or so that the Pokemon can be happy (Butterfree and Pidgeot were left so they could make a family and protect a flock of their bretheren, respectively).