Isso apenas repete as informações da resposta do @ DVK, mas as torna mais explícitas. Em uma atualização de Pottermore, ficamos sabendo que Lucius escapou da captura se entregando a outros Comensais da Morte (não diferente de Karkaroff na primeira guerra):
Abraxas’s son, Lucius, achieved notoriety as one of Lord Voldemort's Death Eaters, though he successfully evaded prison after both Lord Voldemort's attempted coups. On the first occasion, he claimed to have been acting under the Imperius Curse (though many claimed he called in favours from high-placed Ministry officials); on the second occasion, he provided evidence against fellow Death Eaters and helped ensure the capture of many of Lord Voldemort's followers who had fled into hiding. His son, Draco, was saved by Harry Potter during the Battle of Hogwarts, and currently resides at the family estate in Wiltshire.
— The Malfoy Family, Pottermore (Transcript)
Narcisa mentiu para Voldemort e garantiu que Harry sobreviveu para acabar com Voldemort, o que provavelmente seria suficiente para mantê-la fora de Azkaban.
Draco era um Comensal da Morte, mas pode ter evitado a captura por ser muito jovem. Alternativamente, acho que Harry poderia ter reconhecido que Draco estava agindo parcialmente sob coerção, não completamente de livre arbítrio, e poderia ter lhe oferecido uma segunda chance. Citando minha resposta para O que aconteceu com os Comensais da Morte? :
JK Rowling said in a 2007 interview with NBC that Harry and Ron “utterly revolutionised” the Aurors, and presumably Ministry justice systems. In Draco’s case, I think Harry saw the capacity for remorse, and might have been willing to offer Draco a second chance.