Because it has been shown over and over and over again that you can motivate more people to vote against someone (or thing) than you can get them to vote for. Also it is a proven behavioral response that if you disgust voters enough, they won’t go out to vote at all.
So our system these days is all about suppression and discouragement. We need serious reform where voting is mandatory and paid holidays are declared for voting. And that would be just the start of reforms needed.