This is a known plot hole. Unfortunately, any answer requires some speculation.
There are several theories, the best of which is that the Borg, aware of the time-loop created by Star Trek: First Contact, were attempting to keep the timeline intact up to that point, with a plan to attack in force at a later date.
There is also a theory that after the events of Regeneration the Borg sent their nearest ship to Earth in an attempt to quickly kill off the nascent Federation. Bear in mind, several hundred years worth of travel may not be out of the ballpark for a ship travelling from the Delta Quadrant, assuming the Borg were at a lower level of technological development when they received the message than they are by Q Who. This is especially the case if the cube is stopping along the way to assimilate useful technology and replace its used-up drones with new stock, as may be implied by The Neutral Zone. The follow-up attack may have been the next-closest drone.
It's also highly-likely that the Borg, while considering the Federation both worthy of assimilation and a possible threat, have more problems back home in the Delta Quadrant. After all, it is not unheard of for the Borg to bite off more than they can chew, and there are other species in the Delta Quadrant that could pose significant threats to the Borg. It is probable that the Borg only act in concert, as in the war with Species 8472 and the assimilation of Species 116, when they see a considerable threat. Humans don't yet qualify.