—Pichu whenever you finally kill one of those rodents
It was a clone of Pikachu in Super Smash Bros Melee, and it was both weak and pathetic, even by clone standards. It got near universal hate, and was moved to being a trophy in Super Smash Bros. Brawl. It has fans, however few they are... until the confirmation of Dark Pit. The fact that Kid Icarus got another newcomer who doesn't even have a gimmick to distinguish himself from the fact that he's a Pit clone means that Pichu's fans have actually gone up since it actually has a gimmick to differentiate itself from Pikachu.
Plus it did rep Gen II.
And it can actually be pretty good if you know how to use it.
Basically, support for Pichu has gone up because people have stopped spamming B-attacks, Mewtwo makes further DLC a possibility, it represents a generation of Pokemon that currently has no playable characters (read: is not Generation I), and it's not Dark Pit.
Users Who Are Awaiting Pichu's Glorious Return in Super Smash Bros. for 3DS and Wii UEdit
The Return Edit
Pichu, along with literally everyone else, is returning as a fighter. Nobody not even Pokémon fans really wanted him back, though. He's only back because it would be odd if there was just one single character that wasn't brought back in Ultimate.