How?
ow? Supergirl's ratings have always been low, and surely you can sense that the show is winding down. You had one longtime series regular leave this season, and I would not be shocked to see another depart sooner or later. That, and Benoist probably has a desire to do other things, not stay in that costume for another two or three years.
About the ratings, I know that Supergirl's ones have always been low (although the S1 aired on CBS pretty well worked even if the broadcast felt that compared to the financial investment and the audience generally obtained by its series, Supergirl was weak) and the quality goes down season after season but come on, CW and his big brother, CBS, had to set a limit not to be exceeded, right?!
In France, when a program or show is produced or purchased by one of 5 main broadcastings but they don't get the
expected success, the said braodcasting gives it to one of her little sisters (like CBS with CW) but audience limits are being set on every channels, regardless their importance in the french audiovisual landscape). If the ratings reach this minim in terms of acceptable limit, the program is in first, changed time slots and if it keeps slowing down, despite already changes operated such as a change of cast and storylines; the decision to terminate the said program is taken. I don't think that in USA or elsewhere in the right, it is different, right?! Look at shows which stopped after 1 or several seasons with a sloppy end - when there isa end - because the producers did not expect to have to close the doors, letting viewers desesperate because they will never know the end of the story for their favourite characters.
I already know that Supergirl reached its 100th episode while the season isn't over yet and s6 was already announced but if ratings had to reach 500 000 viewers and even below, with less than 0.2, I don't think that CW but even more, CBS would allow Berlanti and his team to keep going without making significant changes to the series, like downgrading regular actors to recurring and even, getting rid of some not very popular with fans or costing too much to production, focusing on 2 or 3 storylines but no more, especially those which go nowhere.
Though the ratings were strongly low, lower than previous last 3 seasons, I think that s6 was already announced because of Benoist's contract, which had to last until next season. But if the viewers keep stopping to watch and/or critized theidiocy of the
storylines offered, I'm not sure that a eventual 7th season could be easily gotten by Berlanti. But it is just my opinion.