I suppose it was too much to hope for an in-universe explanation.
How come the weapons the Borg fire on the Phoenix compound are so wimpy? Same reason.
How come the Borg come to Earth and fight through 24th century Starfleet defenses before going back in time, instead of going back in time first and then going to Earth with nobody but 21st century defenses in their way?
Most depressing line in Star Trek: "something called tequila." No tequila in the 24th century?