Assertiveness is not popular in part because it is not populous.
In most agencies of social control, from hospitals to prisons, people are told to get in line and not make a scene. In schools, students are taught to behave, do what they are told, and not make waves. This goes for teachers and administrators as much as for students.
The quiet students are rewarded, the loud students scolded, and the "incorrigible" cases cast out or they drop out voluntarily.
From cradle to grave, society discourages assertiveness. Yet assertiveness is in perfect concert with the will of God. The martyrs, from Stephen the Christians of the Middle East do not sacrifice the faith of Jesus Christ, even in the face of persecution and execution.
I was shamed by people in my family for speaking up. Like many, they were afraid that I would get hurt. Or they were deep down ashamed because they also wanted to speak up but were afraid to or did not know how.