Mostly it is due to the glasses and the effect the glasses have on the wearer.
Having recently seen my first 3D movie at a theater last night, I can say that yes it does look incredible, but I have significant eye strain, that is still bothering me the next day.
Others I have talked to said they get headaches from the 3D glasses, others just hate having to wear them due to comfort, interfering with their normal glasses or not used to wearing glasses..
Sorry, no one I have talked to is willing to veg out for an hour or 2 in the evenings with 3D glasses on.
I am really not willing to do it for games either. I'd rather have a few hours gaming in 2D, than a short duration with headaches in 3D.