In short, they were on a failing network that didn't have access to 40% of the market that TNG & DS9 had.
They missed out on a lot of folks that could have "cared" about Enterprise.
That was something I could never understand, either. In Canada, in order to be able to watch it, you had to subscribe to a higher tier of programming. This alone shut out many potential viewers. Although it does appear to be more common now, I just didn't know anybody back then who would subscribe to a higher cable package to get one show.