Actually, I think Nintendo doesn't usually reuse their old characters because they can(probably should) make new characters instead. Imagine if Super Mario 3D World used the kings from Mario 3 instead of the Sprixies, the Koopa Kids instead of the new bosses, and even brought back some Paper Mario characters for it, it wouldn't work well because those characters were made for one game(the Koopa Kids would've just been horrible overused if they added them again) and it would mostly likely cause Nintendo to have to adjust the game to fit them in and limit what they can do to them, rather than adjust new characters to do anything the game requires and create an overall new feeling as well as less confusion for newer players, this is also another reason why Mario doesn't have much of a personality or anything, so Nintendo can keep using him in many games while making them very different(which is what Miyamoto intended from the start), Luigi on the other hand, has been somewhat limited to Ghost, Mansions and being scared all of the time, so if Luigi did anything really different, people would say "This is not Luigi!", while Mario is not truly attached to anything so he can change to do almost anything they need him to do and still be correct.
I think most of this applies to Sonic's series as well.
I think most of this applies to Sonic's series as well.