Supergirl (season 1 on CBS) was beyond stellar. A girl power show everyone could cheer for. Supergirl was awesome but vulnerable. A girl next door type or kid sister type. Fallible, but compassionate.
Then...
I was among the fans who bombarded CBS feedback lines to renew the show at the end of season 1 when the fate of the Girl of Steel was not so sure. When the show was renewed but sent down to the piggyback network the CW, I was convinced that with 150% from everyone involved with the show, CBS executives would see their blunder and return the Maid of Might to CBS prime time.
But...
Less than 100% was given by all (especially producers). And junior producer / writers were give free reign to promote / push their personal beliefs on everyone. The thinking apparently was the sooner they could sink the show the sooner they'd be contractually free from it.
And...
The show got worse and worse with each season... producers were no doubt flabbergasted that they could not crash and burn the thing. They underestimated the momentum of season 1 (which carried the show to a sixth season despite how bad it had gotten).
Perhaps when enough time has past and the bad taste in everyone's mouth is forgotten a stand alone Supergirl movie could be made.