It is a mix of tradition and finding members of their own 'stripes' more attractive.
Take me for example. I like red or brown hair, but don't like blond at all. I find certain shades of skin attractive, others not at all. And I don't like the look of Enkis at all, no matter what colors they have.
Tradition fits in the same. Parents can have nothing against other groups of people, but still naturally and even instinctively want their children to grow up and partner with someone who looks more like themselves. Tradition puts a face on that. How many times have you heard mention of a Southern person saying they want their son to "Marry a nice Southern girl."? It is not that they don't like 'northerners', it is ingrained into their beliefs that things are just better and meant to be that way.
I would like to point out now that each pack has different traditions. They are not all the same. Some packs may be more aggressive minded, others might be partial to higher society. That plays a large role as well.
Lastly, there is the factor that if someone partners up outside of their own 'group', that group will often see that as saying that person thinks they are too good for them, thus putting pressure to stay inside the group.
Rich marry the rich, poor the poor, this to this, that to that, stripes to stripes, and spots to spots. Right or wrong, that is the general way of things.
