Re: Why did spock end up in the alternate universe?? The in-universe reason why they don't do it is because they have the Temporal Prime Directive, Temporal Investigators and in later centuries Timeships that undo violations of the timeline. Nobody would be allowed to stop Sisko from travelling to the Gamma Quadrant, because that was simply the natural flow of things. And nobody would allow Nero to fuck up the past over a period of 25 years and kill 6 billion people. Somebody on one of the timeships would notice (since this timeships are unaffected by the changes), and prevent Nero from coming out of the black hole in front of the Kelvin and destroying her.