Ease of development. Think about it: All the devs have to do is change his color scheme and boom! New character. Another reason is that since Sakurai was involved with Kid Icarus: Uprising, he already has a blueprint of what Pit and Pittoo can do. Uprising also has a ton of weapons so Pit can take one half along with his "iconic" bow, while Pittoo takes the other half. Pittoo is for whatever reason, associated with the Staff weapons in Uprising, so there you go, two birds with one stone.
But there's one thing I don't get: Sakurai Bias. Kid Icarus wasn't made by Sakurai; the third game, Uprising, was. Sakurai made Kirby. So far, Super Smash Bros. has four of Sakurai's creations as playable characters: Kirby, King Dedede, Meta Knight, and Dark Pit. Pit and Palutena already existed before Sakurai decided they should be in. Also, from the first game, they were the main characters of Kid Icarus - Medusa was the main villain in the first game. Pittoo is just a creation from Uprising and there are other characters in Uprising people could have asked for like Viridi, Hades, Pyrrhon, Phosphora - she's an Assist Trophy -, or Magnus.
If you want bias, then all the characters are biased since Sakurai chose them. Snake isn't since Kojima begged for Snake's appearance. Wolf is apparently Sakurai's favorite character.