Because there seems to be some confusion -- if "The Originals" gets picked up to series, that means Joseph, Claire, etc will leave "Vampire Diaries" and join "The Originals."
It doesn't mean they'll NEVER EVER make short return visits to "The Vampire Diaries," but they can't do two shows at once.
"The Originals" is a present-day story, not a flashback. They can't be in Mystic Falls and New Orleans at the same time. That's, like... physics.