Settings wise, races do not mix genetically. When a baby is born, is bears either the race and traits of the father or most likely, those of the mother. Furthermore, logic-wise, as Lolitra pointed out very well, some races are naturally incompatible for breeding purposes.
Now, like some people concluded, there are even more reasons which add to that. Sticking to Enkindukai and packs, although it very well applies to every race, a non-comprehensive list of these reasons are: social upbringing, social acceptance, conformity, traditions.