SMB Dimensions is great!

[Langton]

But I have a question, Why didn't you add Luigi in the game.

Uh, I guess you haven't beaten the main game yet then.
Before you ask about why he's a post-game unlock, I think I was just too lazy to do cutscenes for both Mario and Luigi, which would also require changing the game's story to accommodate for Luigi. As you may know though, he's playable from the start in the sequel.