My Name Is Legion wrote:
With the exception of Enterprise and the original run of TOS, Trek had consistently good ratings.
More people watched TOS on NBC than have ever watched any version of Trek since.
As I mentioned in another thread, raw numbers aren't enough; you need the context to understand their meaning. In the 1960s, there were typically only, maybe, 3-5 commercial television stations in any given market, so the total viewership was only divided 3-5 ways. These days, it's divided among hundreds of channels, so each show gets a much smaller slice of the pie. So even the highest-rated shows today are watched by far fewer people than a low-rated show in the '60s.
Compared to the other shows on the air at the time, TOS's ratings were mediocre. That's why it struggled for renewal every year, why the budget was cut each season, why the third season was bumped from a desirable time slot in favor of a show that had a better chance of doing well.