Enterprise started with 12 million viewers and more then half of that viewership disappeared by mid season.
Season 4 was its strongest season creative wise, Everybody tuned out and only a small die hard base of fans kept watching.
Brannon was scientifically illiterate , It is quite evident in a number of stories penned by him as he continually mangled evolution and misrepresented science . Brannon is a better writer then most people give him credit for, The flak he receives stems from the simple fact he was unqualified to do science fiction, It should be obvious to anyone that writing science fiction should require that one would be knowledgeable and well schooled in science.
From what I understand , Enterprise needed a minimum of 5 million regular viewers in order for the studio-network to continue funding the show, Its first season did reasonably well, Things started to slide in season 2 were by the end of the season it was dipping well below the 5 million mark, By season 3 its average was 3 million and its most viewed episode never even reached 5 mill, The studio had to subsidize the show in order for UPN to agree to renew for a fourth season,That season did even more abysmal and many of UPN's affiliates stopped airing the show .