2D movies started out with silvered screens 100 years ago (that's why we call it "the silver screen" when talking about movies overall).
Why did they use silver? Because silver is more reflective. But it switched over to white eventually because projectors got brighter, and silver had always had the unfortunate side effect that while it was bright head on, it was far dimmer at an angle. Something I STILL believe is true now that 3D movies are typically shown on silvered screens, but I imagine it would be even MORESO if you were sitting watching a 2D movie on a screen put up for 3D projectors. You could be the guy in the middle of the theater and be perfectly happy, or the guy off to one side, and it would all look too dim to you.