Quistis just isn't Irvine's type. It's obvious enough since his favorite is Selphie, who's cute and short and as far as outward appearances go, Rinoa fit that kind of 'mold' more than Quistis did. It's not like Irvine could carry it off anyway >.> The only one who can stand next to Quistis (this is purely my opinion) and not be overpowered is Seifer
Irvine knows Quistis is too damn savvy to fall for his charms. I mean, out of the three main female characters, whose panties do you think it'd be the hardest to get into?
Quistis was also crushing on Squall. She even mentions being jealous of Rinoa.
I'm sure she was happy that this arrangement was made.