Cause the devs said it was for story reasons.
"I'm not into you." isn't a story reason.
Cullen survived the Arishok's sacking of Kirkwall, though. Might be hard to find someone appealing, when your only real experience with their race involved violent castigation of everything you believe in and an utter degradation of a city you were intent on protecting. Hawke wouldn't have been thrilled at the chance to romance a Qunari, either, mind you.
As for dwarves, they lack any chance at magical ability and so Cullen's real understanding of them is severely limited. Not like humans and elves, who he's been surrounded by for so long. What he knows of dwarves comes from very narrow interactions. I might think Oghren is cool as heck but I'm not really sure he offered Cullen some fantastic insights into the beauty of Dwarven culture and society. Kirkwall didn't help -- surface dwarves in Kirkwall were generally members of the Carta. Cullen's tantamount to some member of the law enforcement who's only real experience with Dwarves involves people akin to the Mob, shrug.
I only think that Cullen's story precludes any real attraction for horned Qunari and Dwarves. I mean, it does make sense.