After the war, Draco was a bit guilty for his behavior towards muggle-borns so he decided to marry someone who didn't think of muggle-borns as 'filthy'. Also to fulfill his parents wish, he married Astoria Greengrass who was a pure-blood but was exceptionally better towards muggle-borns. That is why he ended up with Astoria rather than Pansy Parkinson or Daphne Greengrass.( Daphne is older sister of Astoria who was in same grade as Draco but she too, is not mentioned as Draco's friend. Although both Greengrass sisters were in Slytherin).
