Because even with the extra material on the side 35mm still is not 16x9 widescreen. The only way to have TNG in 16x9 is to crop the top and bottom. Using the extra material on the side just means you don't have to crop off as much. Although that extra material might not always be useable. There could be mikes, exposed frames, or any number of other problems with the extra information which was ignored since they weren't using it for the 4x3 TV screen. Of course, realistically if TNG ever gets remastered it will be 4x3 just like TOS-R is 4x3. The real challenge with TNG is having to redo every edit and FX shot since it was all done on video and not film.