Pretty much because around that time certain people who didn't know any better were demanding speech and intricate stories in all their games. So they listened, gave Mario and (ugh) Bowser speech, showed those misguided fools their mistake and took it away in the next installment. Zelda and Mario are very much like Chaplin, give them a voice and they loose a bit of that charm. Now if only they could cut out Sonic's tongue.
