Time travel uses wormholes and I guess that they only have one wormhole to use. That wormhole is just a constant certain amount of time backwards in the past because an Einstein Rosen bridge looks like this:
A wormhole is two points of spacetime folded over like a sandwich and connected by that bridge. That "bridge" is always the same size. So if the the future moves ahead in time by 10 seconds then the place in the past that bridge is connected to also moves ahead 10 seconds. So, if the bridge is "10 years long" then you can only go ahead or back 10 years at any given time.
So you can only go to 1900 from 1910. Once 1910 becomes 1911, you can only go back to 1901. Etc.
I just finished watching Deja Vu and it explained it like that.