It's an odd one, really. Nguyen pays attention to stuff like Kate Kane and Renee being the Question, but he uses Barbara as Batgirl.
Part of it is that he really, really likes The Animated Series from the 90's - His characters are all really heavily influenced by that especially his Mr Freeze and Riddler. I think he just likes that series version of Batgirl. He has drawn Cass, as I recall, so... *shrugs*, yeah. Probably just personal tastes.