I know that SooperTrooper probably told you, but most Pokemon Ash catches tend to be heavily marketable stuff like starters. Based on examples I saw, if a starter belonging to Ash never evolves you could basically say it's more popular than its evolved forms.
As for older Pokemon returning, it's probably to remind viewers he has more than just the Pokemon he caught during the current saga, cos each new saga is supposed to show a new generation of kids just getting into the anime without much knowledge of Ash's past.
Also, he has caught at least one Pokemon of every type except Ghost, Psychic and Steel although he did have a Haunter temporarily in Kanto to help him battle Sabrina. The capture of Gible marked the first Dragon-type caught by the main cast, same case with Scraggy becoming the first Dark-type owned by a member of the main cast (Ash and his companions) although Ash does have another Pokemon related to a Dark-type by evolution (Corphish, which never evolved).