Stupid writing. That's always
the problem with why the Borg never win. They made a concept that was essentially perfect in theory, so they have to make the Borg stupid so they could have another episode of Star Trek next week...
I agree. But from a "trek science" point of view, I think the logic is that the Borg don't want to "waste" resources to assimilate someone when there is no imminent need. If they have a full crew, they won't bother spending the time to assimilate (including outfitting the new borg with the electronic appendages).
Though logic would say that after BOBW, the Borg aught to have known that humans on board leads to bad outcomes, and they should have attacked on sight.