I'm in the middle of my fifth or sixth Voyager rewatch now, and I don't even like Voyager that much. It's the perfect sort of thing to watch as I drift off to sleep, and the quality really improves after Year of Hell.
Others have brought up other reasons, but one I haven't seen is simply the depth of material to work with. Trek has had multiple multi-season series and tons of movies, which means you can basically find the right image for any meme if you watch enough of it.