Because Lucius Malfoy was a bully himself. And like all bullies, including James, Malfoy was a coward at heart.
Young people often are cowards, in that sense. Rather than being bullied by their peers, they prefer to be the bullies themselves. So if you think about it, among that group of young Hogwarts students--James, Remus, Sirius, Peter, Lily, Severus, and Lucius--if you think about it, Lily was the only one among them who was not a coward in any way.
But as people mature, they usually grow out of that stage. They learn how AND WHY to stand up for others. James, Sirius and Remus all did. Peter and Lucius Malfoy did not. That's why they both gravitated to Voldemort: they were looking for an even bigger bully to hide behind.