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.
Though I am very happy with what we got. I used to find it depressing that it was cancelled, I'd watch a really great ep or pretty much all of season 4 and it would be this shadow of sadness on the whole thing. But now I'm just like, HEY GREAT TREK!
They could have cancelled it after season 1 and that would have been