The quality likely has little to do with it. People were simply burned out on Trek. You can only go to the well so many times. I was a huge Trek fan, and every new show simply became "more Trek". I started tuning out.
I think it's more the case that each successive series was really only appealing to the existing Trekkie base. Add to that that they went through their own decisions to "shake things up" which really just alienated a portion of the people who started watching TNG. Things got worse for ENT, as a "third generation" show, as few people who tuned out DS9/VOY would give it a chance, yet invariably it didn't appeal to everyone who liked those shows either.