I thought it had to do with half-breads. Since you can't do that cross-species thing. The truth is that you should probably not allow half-elves or half-orcs. Elves are an alien species that somewhat resemble humans, they should not be able to have babies with humans. Look at lions and tigers, they are both feline but the combination are sterile or worse.

A lot of it is also do to Tolkien, he says the race of Elves and race of Men or something to that extent. He also made half elves.