Of course theaters that don't have 3D projection equipment would show it in 2D.

As for the theaters that have both, the 2D showing provides additional income without the necessary investment in 3D equipment. And there's always the repeat-viewing factor: I saw it first in 2D (quite enjoyable), but then decided I really had to see it in 3D (as Ed says, a revelation). So they sold me two tickets.

Eric