It was originally shot to be 4:3 if they did 16:9 they'd either have to do one of two things:
1. Simply use a wider film exposure. However, this may reveal crew, equipment, and other elements of production just off camera.
2. Crop the picture. This would remove elements from the top/bottom of the picture which would alter the original episode and director's intent for the scene.
The show was show shot to be seen on a 4:3 screen and that's the way we should watch it. People need to stop this obsession with "filling their screen" with a picture.