The way you succeed in anything, business, politics, anything, is by keeping your base and adding to it. ENT alienated hared-core fans while failing to attract new ones in significant numbers.
Whose fault? Gotta say the suits. The proof, season 4. The suits had left the building by then, having decided that ENT was dead meat and that they only need enough eps to round out a syndication package. And in that atmosphere of benign neglect, the show flourished creatively. Oh, season 4 had its stinkers, to be sure. But if the entire show had been as good as season 4, it would have gotten seven years.