That is true, 2D Mario games are huge sellers. In fact, I think it's still the highest selling Wii U game to date.I thought that too, but the 2D games sell more and are more mainstream. 3D is more for hard core gamers, I figure they would receive Luigi better than the casuals.
But you're right, Luigi already has his own games.
Truth is, I just wish Nintendo would share more of this AAA talent with other IPs, new or just neglected like Kid Icarus.