The 3-D systems from years ago didn't work very well. To understand exactly why, you have to know how light works.
Light is a type of wave, and those waves move up and down and side to side and basically all over the place. Light is a bit chaotic. A polarizing filter blocks many light waves but allows those moving in a direction parallel to the filter to pass through. In other words, a polarizing filter cuts down on visual noise.
This is readily apparent if you take a pair of polarized sunglasses and rotate them while looking at a bright surface with a lot of shiny, reflected light. At the right angle, the glasses will block much of the glare and make the scene easier on your eyes.
Engineers can make eyeglasses with lenses polarized differently for each eye. That is, one lens blocks certain light waves and the other blocks a different set of light waves. That's ultimately how 3-D theater technology works. And we've come a long way since the early days.
Old-school 3-D projection technology was a headache, literally, for both theater owners and for many viewers. For starters, these antiquated systems required not one, but two projectors, which cast two different linearly polarized images onto the screen from 45 degrees left and right of center.
Viewers wore linearly polarized glasses, which had to rest at a precise angle (in other words, straight on and stationary with respect to the screen) in order to present crisp images. Thanks to the glasses, your right eye saw only images from the right projector. Your left eye saw images only from the left projector.
It sounds workable in theory, but if you tilted your head, you'd see colors bleeding into each other and stomach-churning, distorted images that gave many movie-goers headaches. Other problems arose if the angle of the projectors was off even slightly, the images weren't perfectly synchronized or if they weren't exactly the same brightness. It was an altogether frustrating experience for projectionists and audiences.
Fortunately, systems featuring linear polarization are no more.