Why doesn't Ash use his old Pokemon? Why doesn't Ash uses any of the Pokemon he caught in previous regions, apart from Pikachu? We only seem him use Charizard in Best Wishes and a few examples in Diamond &amp; Pearl (only so far though because XY&amp;Z haven't finished). I get how he wants to start fresh with each new region and use new Pokemon to beat the league (to make it more adventurous), but is there a specific reason for why he doesn't use them?
From S05E275 (&quot;Gotta Catch Ya Later!&quot;, second to last episode of Master Quest)

Ash: Professor! I've decided to start a new journey too, and only take Pikachu with me. I want it to be just like the first time I left Pallet Town.&quot;
Pikachu: Pikan~~
...
Tracy: And you can count on me to take care of your Pokemon for you!
Ash: Really? Thanks a lot, Tracy.

Ash wants to experience the thrill of discovering the wonderful world of Pokemon all over again- a feeling he knows he won't experience if he only relies on his old Pokemon.
He remembers how much he loved the experience of being introduced to a whole new realm, and how he had to work at leveling his Pokemon and growing with them to be able to eventually win the Orange League Championship.
He decides to leave his Pokemon so he is forced to level native Pokemon, knowing he will grow as a person during this process.
He leaves his Pokemon at Professor Oak's Laboratory because he knows they'll be well taken care of.
Throughout the Pokemon series, Ash only ever brings Pikachu with him to each new region, as Pikachu is his best friend and they are inseparable. The are two execptions to this rule: the Johto region, due to behind-the-scenes expectations (Pokemon was originally only going to have Gen I and Gen II), and Diamond &amp; Pearl, where Aipom sneaked aboard the boat Ash took to the Sinnoh region.