Because it ultimately leads to loss of diveristy, ironically enough. I like sseing people with brown hair, blue eyes, red hair, green eyes. I like seeing pure Asians, Indians, Native Americans, Whites, Phillipinos, Turks, Mongolians...Ect. Ect.
Yet, if you have mixed race children, and it goes on for a mere four generations, you lose all that separate beauty and are left with just ONE similar looking race. Brown eyes and Black hair ad infinintum. Just look at northern brazil to see the end result.
So yeah, whats wrong with any of that?