In order to get 3D you have to get a different image to each eye. Just putting a lens in front of a screen would not be sufficient unless that lens somehow was tied to a viewers individual eyes.
I don't know the technical details, but the way I understand it that it is similar to privacy screens for computers where only the person directly in front of the screen can see and people peering at an angle will see black. Now narrow that perfect angle down so only one eye can see the image and add another filter for the other eye. The 3DS has a huge advantage over an entertainment system as it is easier to guess where the eyes are, and it only has to deal with one left and one right. In a home theater set up there would half to be a parallax pair for every viewer.
"You can never jump away from Conclusions. Getting back is not so easy. That's why we're so terribly crowded here."
Canby - The Phantom Tollbooth
