Pichu wasn't cut in Brawl. When he was told he was going to come back for Brawl, he was filled with immense happiness and evolved. Pichus often evolve to Pikachu if they get enough happiness. Sakurai, thinking that two Pikachus was not necessary and that the Pikachu from Melee was getting on in years, decided to retire the Pikachu. Getting its retirement money, The Pikachu from Melee went to Cinnabar Island. (Pokemon version of Bahamas).
How do we know this? Pikachu's Final Smash is Volt Tackle, which can only be learned if you were a Pichu in your previous life. Also, Pikachu (Brawl) now sports blue goggles, its favorite costume from Melee, along with a new green bandana that it took from Jigglypuff.
Another thing would be Pikachu's Taunts. It keeps the same rolling taunt from Melee (altered slightly) along with learning the former Pikachu's taunt. It's moveset and the way it moves and feels, feels like a combination of Pichu + Pikachu gameplay from Melee as well.
Young Link wasn't either. How many years have passed since Melee came out?
Seven. How old is the difference between Young Link and Adult Link?
Seven. He merely grew up.
^That would make a perfect avie. <_<