Im not quoting anyone, i am paraphrasing the people who try to stop diversity.
Actually what you're doing is trying to start an arguement, no-one I've seen has been trying to stop diversity. The real world contains constraints on ethnicities. If a native american showed up in England in 1250 people would have some questions.
The reason white doesn't need an explanation is because Bioware themselves made white the default, which makes sense as Thedas is based loosely off medieval Europe. I'm all for adding ethnically diverse characters, but please make them make sense with the universe Bioware has established.
I know right? If ___ years in the future we decided to take the population of one North American city on our fancy spaceships and colonize another world, that new population would contain multiple races. Their descendants wouldn't be asking for explanations as to why not everyone looked the same, it would just be how humans are. With a video game, especially a fantasy video game the creators can do whatever they like. I don't feel the need for them to create an entire geographic and evolutionary history for humans on Thedas just for people who don't look white to be included. Human ethnicity isn't even an issue in Thedas so why does it require an explanation for so many?
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Because Thedas wasn't colonized from a spaceship (I very sincerely hope Bioware never, ever goes down that route. Far too many jrpgs do that left, right and center)
It requires an explanation because we find adding realistic aspects keeps us immersed in the world. If a cat-person suddenly appeared, no explanation given, they were just there, and no-one batted an eye that would be immersion breaking (every time I watched dbz as a kid those animal people always jarred with everything else, even with all the space races stuff). In a modern world we don't need excuses for why people of different ethnicities live in different locales, but Thedas isn't a reflection of the modern world by any stretch of the imagination. And human ethnicity is indeed an issue in Thedas, but it is mostly based on nationality rather than skin tone (historically accurate since most of Europe was the same general white skin tone, so most 'racism' was more along the lines of french hating germans, etc). The Orliasans look down on everyone else, as do the Tevinters. Insults like 'Dog Lords' wouldn't exist in the universe if ethnicity wasn't an issue.