The original 4;3 (or 1.33:1) ratio was chosen for technical reasons, because it was the optimal ratio for the width of the film that was being used at that time. This ratio was eventually chosen as the standard, and when televisions appeared it was logical to have the screen conform to the standard.
the 16:9 ratio is actually a compromise. It allows the viewer to watch wider screen movies in a wide screen while still displaying 4:3 pictures. It is not coincidence that 16:9 is actually 4 squared : 3 squared