In the latest Iwata Asks feature, the Super Mario Bros. team discuss why Nintendo chose Luigi and Nabbit to feature in the DLC pack.
Initially Nintendo were to make a straightforward DLC pack by simply "changing all 82 courses", but director Masataka Takemoto wasn't fully convinced. By changing the lead character from Mario to Luigi.
As for the game's multiplayer feature, Producer Takashi Tezuka was "absolutely against having Mario make an appearance", something unheard of in Nintendo's EAD department. At the start the team were to use three toads, but "the screen gets confusing" and so the "only choice left was Nabbit". Because Nabbit doesn't take enemy damage, "the programmer was less than thrilled" in the concept initally, but became more confident when making the prototype.
Nabbit is also available as a single player character, by holding B before entering a level, but Nintendo chose to keep this secret as a throwback to "hidden commands" in older games.
What are your thoughts on the absence of Mario and playable Nabbit in New Super Luigi U?