Lack of diversity
Rather than wondering why Numenor is improbably diverse or why is there a black-skinned Elf, let's ask why Southlanders are all "white"?
I mean, unlike the Gondorian "colonizers" they are native to the South. And we do know exactly which humans fought for Morgoth. These were Easterlings - inspired by our Earth's nomadic people of the East.
So they should either look like Mongols or similar to Iranians.