Enterprise aired on UPN, Paramount’s now defunct attempt at a fifth broadcast network in the U.S. They didn’t have enough stations to begin with and didn’t have them in key cities that figure heavily into tv ratings. (The Nielsen ratings system was an inaccurate joke even when TOS was on NBC, but it’s what the industry swears by to estimate what people are watching.) I knew fans who were downloading the show as they had no other way to see it. Online viewing was not factored into viewership at the time.
UPN fell apart not long after Enterprise was cancelled at the end of season 4. More people probably saw it in reruns on the Sci Fi channel.