Probably said already but it was confirmed that if two races interbreed, 90% of the time the child born takes after the mother. So if it's a Hyur father and a Miqo'te mother, the child born is Miqo'te. Only under rare circumstances does the child become a hybrid, the most drastic case we've sen so far is Hilda during Heavensward. This is why we don't see so many of them walking around. It's a case of it being rare. The reason Hilda stands out is because her backstory involved nobles and commoners, so there was no hiding what happened between her parents.