It is possible that the Borg had just recently invented/assimilated the time travel technology and it was not a sure thing. If the technology was new and not completely refined, perhaps they only had enough power for one time travel trip.
But that happened in 2373, and we know the Federation is still intact as late as 2387. If the Borg got time travel once, then presumably all Borg would've had the knowledge from then on, and could've built another device. So the galaxy would still be screwed.
Giving the Borg time travel at all was a very bad idea storywise -- it opens a huge can of worms that the screenwriters didn't bother to address because it was nothing more than a plot gimmick to them. It would've been better if they'd had the Borg and the Enterprise
thrown into the past by accident -- say, "Yesterday's Enterprise" style, with the energies of the battle interacting with a natural cosmic phenomenon and creating a time rift.