DC had a strong preference for Wally, until they didn't. Babs being the only Batgirl the public knows doesn't matter much if the powers that be think that there's money to be made elsewhere. Remember, Cass was the first Batgirl with her own series, and it's still the longest unbroken Batgirl series around.
As for Damien, he really doesn't fly Cass' colors. If anything, they act as if the nature vs. nurture debate with him is settled since being Batman's son automatically entitles him to be Robin (somehow), his ability to function in society is hampered only by his ego and while he might technically be bi-racial, he still somehow manages to look like all the other Robins that came before him. In addition to that, he has no real redemption arc.
At best, Damien is Cass-lite