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 & Pearl (only so far though because XY&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 ("Gotta Catch Ya Later!", 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."
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 & Pearl, where Aipom sneaked aboard the boat Ash took to the Sinnoh region.
From producers point of view, Ash using new Pokémon means that those Pokemon got time to shine. Sure they can have Ash use his old pokemon, but there are 2 problem with this. First is there will be less time for new pokemon to show their skills which translates to less time for them to attract people to buy their merchandise. Second is that if the old pokemon lose to the new pokemon, old fans would get upset.This was the case with Kamen Rider tokusatsu series where fans of Kamen Rider Kabuto would get upset at Kamen Rider Decade (the character and the series) due to Kabuto losing to Decade.
From Ash's point of view, he collects the pokemon. In battle there is always the risk of his pokemon getting injured or worse killed. Sure he can just go back to the previous area to catch it again. But this will cost money and time. Using new pokemon that live in the region he is in means that supposed that the pokemon got killed in combat he can capture another with less time and money spent.