We have to also consider the fact that 3D very rarely actually adds to the experience of watching a movie (Doctor Strange is a recent exception, but those exceptions are few and far between).
A big part of that is that 3D requires the wearing of specific glasses to fully experience the image onscreen.
For many people, watching 3D movies leaves them with headaches or in extreme cases nausea.
These glasses are annoying to wear while just sitting in a movie theater; who wants to wear cumbersome glasses while in their own home?