She left a similar lifestyle behind, suggesting that she dislikes it (or perhaps the people even).
Or it just wasn't for her. She never states any kind of distaste for noblility. Afterall, she is a distant cousin of the royal Pentaghasts, not a princess. She would have been rich, but not abundantly so. And she was taught to fight at an early age, given her dragonslaying skills. As she got older and embraced her growing faith, I doubt the life of a noble appealed to her at all. But this does not mean she automatically hates those who like it.
Besides, the human Inquisitor is one of two things:
1. A Circle mage, something that Cassandra has indicated quite clearly that she can feel some level of affection for.
2. A noble now in service to the Chantry(whether they like it or not).
Neither of these indicate that they are a typical noble, and therefor something that Cassandra would innately dislike. In fact, Option 2 implies a great deal of commonality with Cassandra.
So as nice as it might be to have the humans racegated for a LI(And I do think that will have to happen it Varric is a LI), Cassandra is almost certainly not going to be the one they can't romance. In fact, they are the only ones guaranteed, IMO. I highly, highly, highly doubt that Cassandra is racegated, and certainly not for humans. I can see reasons for any of the other races, but not the humans.