Because punishment tends to immediately terminate the punished behavior, while not preventing it in the long run, punishment reinforces the punisher. The person doing the punishment is an agent under conditioning as well, but most folks fail to notice this.
Upvoted for the insight.
while not preventing it in the long run
I don't agree with this, but rewarding is clearly better, of course. It's easy to get tunnel vision when thinking about punishment, many of us have memories of injustice done in its name.
Related: Son of Low Hanging Fruit
Another post on finding low hanging fruit from Gregory Cochran's and Henry Harpending's blog West Hunter.