The whole slingshot around the sun to time travel business has to be forgotten by necessity in order to have dramatic consequences. Otherwise, they should have changed a lot of things post-"The Naked Time." The other alternative is that those who try to change history are frequently prevented from doing so by future timeships or some other agency. We really don't know if Spock tried to prevent Vulcan's destruction or not. The movie had to end, and that consequence had to stick in order for the galaxy to be an unpredictable place again.