I'm pretty sure its part of the synergy/hype for the new DC universe movies. Also, theres been other Supergirl projects that have gotten off to false starts. Its weird but even this series' premise sounds more suited for the character Power Girl than Supergirl, I'm only guessing they're using Supergirl in this instance mostly for name recognition.
When theres no more room in Hollywood, remakes shall walk the Earth.

so the only movie in which supergirl has the first lead role is the non animation 1984 flick with helen slater.
All the other live action films // series supergirl just appeared briefly on smallville and in those animation flicks u mentioned.
I wonderin what grounds they are doing a tv series live action with supergirl as a main theme when they have not done anything like this in 30 years time..LOL

Supergirl plays a major role in the DC animated films: Superman/Batman- Apocalypse and Superman Unbound.
She also has an important story arch thoughout Justice League unlimited season one. And she appears in key episodes in season two.
When theres no more room in Hollywood, remakes shall walk the Earth.