I think there may be some mix of greater quality (or at least some context to help explain why that’s percieved to be the case) in the Steph and Cass stories with some element of originality and “osmosis” of quality from the time period they were being written in.
Cass’s story launched when the Batbooks were still on the upswing from NML, and when a guy like Chuck Dixon was still the workhorse for the “B-books” in the franchise... but doing a stellar job at it. And it also lasted through three years of Gail Simone’s first real run on Birds of Prey. The perception of quality in those books, peers to Cass’s Batgirl series, is going to reflect positively on her series when she stands alongside them. Plus, let’s face it, if you’re a fan of Cassandra Cain from back then, it’s 99% Puckett’s work that made that happen - her fanbase is going to be happily tied to that run by its very nature, much like how Tim Drake fans are tied to Dixon’s work on Robin, or how BoP fans are tied to Simone.
Similarly, Steph’s book launched during the late Batman renaissance of RIP-Inc, where Morrison and Snyder were juggling the big books people were reading... and where names like Paul Dini, Simone again, and Fabian Nicieza were running B-team books. So Steph’s book received a similar rub to Cass’s. Bryan Q. Miller also got to make the most of her run, managing to win over fans and make the book a true cult classic at minimum. He also was writing Steph in the ID for the first time, so again, BatSteph fans are always going to have a high opinion of him.
Babs has a different set-up for the modern era.
Her run launched in the shadow of Oracle having been defined by Jon Ostrander salvaging a diamond from the sludge of TKJ, with people like Dixon and Simone polishing and honing the diamond for decades, and with multiple earlier Batgirl stories published by different people. The fact that the New 52’s B-team Batbooks had a more rocky reputation doesn’t help, and neither does the fact that Simone’s run on a Babs in the Batsuit didn’t take off as expected.
I think there might be some quality arguments to make... but I’ll also confess that I’m a bad source for it, since I just tend to shrug about Babs as Batgirl now. The fact that she feels “lesser” to me in the suit and not as Oracle, and that her book had a bit of chaos early on means that I just don’t care about her current run that much.
And I think that perception is a problem she’s going to have until she has a sustained long term run for a while.