In no particular order...

1. Many men are total pussies and would never consider self defense to the point of killing someone something they would do.
2. Somewhat contradictally, people don't trust themselves with a gun, so therefore, they don't trust others with guns.
3. Feminism. Feminism raises mothering to a societal level. Women want everyone to be protected and safe via the a motherly govt overlord. Women intrinsically desire equal outcome. That requires communism. So any means to achieve that. Like authoritarianism.
4. The rise of Marxism and authoritarianism. Many people want to be told what to do. That only works on a broad scale if the people who refuse are unarmed.
5. Ignorance. Cognitive dissonance. Narcissism. Lack of critical thinking skills. Inability to set feelings aside and consider things logically.
6. Brainwashing. There's about 3 generations that have grown up with commie public schools telling them guns are bad. I've converted quite a few democrats from gun fear to fun by just taking them shooting and covering the $40 worth of ammo they use up.