One possible answer is that they don't consider it worth more than one cube at the time, since Earth is not an enemy that can threaten or seriously damage them. (Apparently a miscalculation after Janeway arrived on the scene.)
Another theory is that the Borg are milking us. That is, they just send over one ship. If it succeeds in assimilating the species, fine. If it doesn't, said species will feverishly try to improve their defenses and invent new technologies, that can then be assimilated. Repeat a few times, or for a few centuries, until you have tapped dry their potential (they'll invent no more new tech the Borg don't already have) or they approach the technology level of actually being able to resist successfully. Just short of that point, move in with hundreds or thousands of cubes and start harvesting the entire species.
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