It's short and to the point. But there have been at least two "bring back Cass" campaigns I'm aware of the writer doesn't seem to be. That's cool. After all, I couldn't tell you how many "bring back Steph" campaigns there have been. Probably more than two.
My gut feeling is Steph fandom is much more active and vocal lately than Cass fandom for various reasons. For one, I think the character is simply more popular. To list a few others, Steph's stint as Batgirl is more recent; the character headlined a book wiped out in 2011 by the New 52 while Cass' last book was that abysmal Redemption Road miniseries way back in 2008 (which with all the reboots and retcons is the comic book fan equivalent of about fifty years of regular human time); the character has a longer history than Cass does (Steph debuted in 1992-- about 1000 years ago in fan-time-- while Cass didn't make the scene until 1999); Steph was a Robin until they tortured and killed her (as a controversial story point, this tends to trump a simple drug-induced heel turn), and so on and so forth. I'm tired of typing comparisons now.
It might also be Steph is more accessible a character than Cass for a lot of readers. She can verbalize her feelings, she's usually written as witty or smart, the kind of character you instantly identify with and root for. Or, at the very least, find fun to spend time with or read about. Even as a staunch Cass fan I can see how a silent, former killer who seems untouchable at times could be off-putting or even alienating to some readers (although I can't see anything about her that's particularly clichéd).
Anyway, we see which character DC has picked to debut first in the New 52. That's also fitting for a lot of the reasons I've already written about here. But like the Steph fan we started this little rumination with, I can't see a single reason why Cass should remain on the shelf gathering dust for much longer.