^The music had a lot to do with that. By the fourth season, most of the musical scores had lost their bite. Melodies were gone, and much of it felt like filler. It made a lot of episodes feel plain.
That was a big part of it, but it wasn't just that. Everything was familiar. There wasn't the sense of being out in uncharted space. It was just Klingon stories, Romulan stories, and going from one starbase to the next. It got kinda dull. The dull music didn't help, of course.