One of the factions detonated some sort of chroniton weapon in the part of spacetime that corresponds to the latter half of the 31st century. Since then (and before then, too, since it works that way), no time travel has been possible from points before it to points after it, or vice versa. The only way to get past it is to live through the time it occurred in, in real time. It might be possible to time travel forward to just before, wait, and then jump further forward after the explosion - but you could never return, which makes it mostly irrelevant to our heroes of the 22nd-26th centuries.