Why there needs be only one timeline

Quantum mechanics say that things don't exist until they are observed; OK it doesn't say that exactly but is a loose translation. So alternate pasts are out. Every choice we make, every event that happens is predicated upon the things that come before it and so even though we think we have multiple choices in fact we will only make one choice, and if an outside viewer were to watch us over and over again we would always make the same choice.

So if you had the choice to go left or right and you chose left then if you were watched making the choice over and over again you would always go left. Whatever brought you to that point has already chosen for you.

Thus there are no alternate timelines. What you are going to do you are going to do and there is no alternative.