It's just a question of aesthetics. TNG was lit almost more like a sitcom, with bright even lighting coming from all directions. In the 1990's, that made TNG look like a TV show, and look very different from dramatic movies.
Then with the rise of TV as an art form rivaling movies, certain dramatic TV shows have been lit more like dramatic, dark movies. Lots of highlights and shadows, instead of even lightning. It's meant to seem more sophisticated, serious, and artful. It also demands that you be watching in more of a cinema-like environment -- a bright, high-contrast TV in a dark room, so you can see the darks. Not a crappy low-contrast screen in a bright room.
But again, this is only certain types of shows. Comedies and "lighter" dramas are still lit more like TNG. It really depends on the show, and what mood the creators want to evoke throughout.