I agree with the oversaturation thing. I remember on Saturday afternoons Fox used to show TOS at 2, 2 episodes of TNG at 3, the new Voyager at 5, and the new DS9 at 6. At the same time, the WB was showing TNG at 6 and DS9 at 7. That would be a bit much to viewers.
Plus, I think with Voyager, UPN of the producers or whoever totally misread their audiences. The franchise was evolving with DS9, the show got really good as it explored darker themes, and had the multi-episode story archs- yet with Voyager, we got TNG mark 2, but with mostly rigid characters and some of the most cliched writing in Trek's history. I enjoyed Voyager when it was on, but I think it was a complete misfire for the franchise. That's when I personally started losing interest, when DS9 ended and we were left with status-quo-enforcing Voyager. There were just no new ideas after that, and fans want something fresh, not rehashed ideas, shallow plotlines, and little-to-no character development.