Peach gets no respect I tell ya, until now.
Koichi Hayashida, the director of Super Mario 3D World, revealed to Polygon that Princess Peach was not originally planned to be a part of the game.
Hayashida acknowledged the fact that Peach has not been playable in main line Mario games since the American version of Super Mario Bros. 2, but that Producer Yoshiaki Koizumi suggested she be included this time around. Hayashida confessed that he thought it would be nice to play as a female in a Mario game after all these years of representing male characters.
For his part, Koizumi wanted Princess Peach back because "she adds a lot to the sense of competition when you get in multiplayer" when players choose different characters to play based on their preferences and playstyles. Koizumi also added that he felt Peach is "really fun to play as."