Sorry, but there is no way Cassandra Cain works better than Batwoman as an outside property. Nobody outside of the DC Comics fanbase know who this character is, heck, I doubt too many outside the Batman fanbase know her.
She has no real hook and there are better characters who tick every box she does, with much simpler stories and a higher profile (both in comics and outside media, such as cartoons and live action TV shows). By your own point, she was Batgirl/Black Bat/Orphan, sometimes she can talk, sometimes she can't, this isn't an easy sell for a character a mainstream audience has never heard of.
Batwoman is a simple brand - female Batman. She has the additional hook of being a LGBTQ character.
Batgirl is going to be Barbara Gordon - Young, female Batman. Has the additional hook of being Commissioner Gordon's daughter.
These are simple concepts to sell.
I will add that Cassandra Cain is a main character in Birds of Prey (and the Fantabulous Emancipation of One Harley Quinn), so she will be appearing in the wider universe.