Probably so the incumbent could run up his numbers, make his victory margin even bigger. A despicable means to achieve a dubious end. All did their share this year. There were probably a dozen or more media consultants producing commercials for the major candidates. As Fred Allen said, imitation is the sincerest form of television.
Candidates use negative ads because they work.
End of story. There is no debate over the wisdom of employing these sucker punches. So it's up to you and me to wonder about their cumulative effect. After all, one small dose of arsenic won't hurt you. A hundred small doses will kill you.