Star Trek tng was shot on 16:9 film, transferred to 4:3 video to be cut and turned into a final 4:3 video. When they did the blu ray remaster, they had to match and rescan every shot from the source film, re-edit each episode and recreate the special effects (when not produced on film). There was the thought to maybe scan the material in 16:9, but the scenes and crew only considered the 4:3 frame - in 16:9 there would often be crew or mics in the frame or even perhaps sets missing. So they kept the show in 4:3.

Apparently for later trek shows (ds9, voy) the film crews were more careful to respect the 16:9 frame, but those shows aren’t as popular and may never get the expensive blu ray makeover.