That is because when Lucas made Star Wars he thought Leia would end up with Luke. There was no romance with Han. But since history has been rewritten that they were always brother and sister, and it was always the Saga of Darth Vader and Star Wars was always episode IV a New Hope, people believe it.
That sure makes the ‘kiss’ in ESB make more sense.
Its true SW as a trilogy, even Saga was fluid, with GL making changes on the fly.