Because he would've made an awesome character, considering his wacky schemes and disguises from the DKC series.
Not to mention his DK's main villain, yet made no appeaerance whatsover in the story mode. It's especially annoying when you take into account that Bowser, Ganondorf, Wolf, King Dedede, and (in Melee) Mewtwo are playable, while Ridley and Porky are major bosses in the SE too.