To be fair according to First Contact its a decent place to amass a fleet to protect Earth from the Borg that isn't that far away from seeing as how fast the Cube and the federation fleet got to earth from there.
First of all, First Contact
takes place five years after "Cause and Effect," so that doesn't affect what was stated in the episode about it being uncharted before that point.
Second, we don't actually know how fast the fleet got from the Expanse to Earth. Consider that the Enterprise
had to get from the Romulan Neutral Zone to Earth in the same amount of time. That should be several days' travel at least. Despite how the film was edited, it's reasonable to assume that the battle near Earth was a separate battle taking place days after the first engagement in the Expanse -- or else that it was a running battle lasting for days.