It could be that every universe is set and unable to be changed. The 'changes' that you perceive from 'time travel' may simply be a shifting of awareness from one universe to another, among your countless 'selves'.
The TNG episode parallels states flat out that a parallel universe is created whenever anything happens.
DATA : "For any event, there is an infinite number of possible outcomes. Our choices determine which outcomes will follow. But there is a theory in quantum physics that all possibilities that can happen, do happen in alternate quantum realities."
So when you time travel and "change the past", what you're actually doing is moving into a different quantum reality. The original timeline is there, you're just not part of it any more.
The oddity to that is how people perceive and react to the changes. For instance in City of the Edge of Forever, when McCoy saved Edith Keeler, he should have gone into the quantum reality where she didn't die. But in that case, why did Kirk and the Enterprise crew know that the Federation was gone? Why did they even exist - shouldn't they have been wiped from reality with the rest of the Federation? Or rather, shouldn't they still be in the reality in which Keeler died and the Federation existed, whilst McCoy had gone into the one where she lived and it didn't?
I guess the Guardian isolated them from the changes. Much as being in the wake of the Borg cube isolated the E-E from the changes the Borg made to Earth history.
What's odd to me is that usually when stuff like that happens, the people involved act like they need to revert the timeline, i.e. return themselves to the Prime timeline. But then Spock found himself in the Kelvin timeline, and he was perfectly fine with it and took the view that it was a nice place to live. No effort at all to undo it or get back where he came from. Odd.