Because they are the mascots of something as oppose to him, yeah he is the evolution of the most recognized pokemon, but that's all. He is one of the few pokemon to be obtained via thunderstone, but Eevee is the staple pokemon for stone evolution.
They should make him, just so Ash's Pikachu will get owned by him and make yet another episode dedicated to evolving intro Raichu and the thunderstone. --- "I noticed your beautiful hair right away" - Minato to Kushina
My favorite electric type, bar none. Dont know it he needs a mega, love hos design now and cant see it being improved really....but maybe. --- "What is great in man is that he is a bridge and not an end"