I don't think it was Draco's or Pansy's decision at all. I think pureblood families are in the habit of making decisions for their children regarding marriage. And I think that Pansy's family felt that after the disgrace of Draco's father being in cahoots with the Death Eaters and Voldemort, however reluctantly, Pansy's parents decided that Draco wasn't a good match for her after all. So Draco's parents had to turn to the next best thing, which was the Greengrass family.