Flippantly, because species isn't race. I'm happy to throw the concept of "species" under the bus exactly because the first question for years has been pick your fantasy race and class. That said, and I know this seems like a mutually exclusive concept, race and class effectively IS background.
And the mutual exclusion works because the question is, given this scenario, would you play this, not would you advocate for it to be a change to the rules overall. The question is, would you accept it as a houserule for a specific game, and what would you play.
Or at least that's how I understand it.