Because there are too many other people who see black people as thugs.
Dude, just go google "philando castile thug". Go ahead. Try it. It's a soul crushing realization.
Many of them won't outright call him a thug, but the narrative is there. He had lots of traffic violations. He's driven with his license suspended or revoked. While not violent, he has a history of being arrested multiple times. He wasn't complying with commands quickly. The officer had more knowledge about both the robber, and Philando, and had more reason to suspect him than what the video suggests. His mother lies about the timeline of the events up to and after he was shot. The girlfriend is a rachety lying bitch. It all comes down to this:
He was a thug that got what was coming to him.
And that's the fucking thing that they do to every motherfucking black man that is shot and killed by police. They dehumanize them. They look up their criminal record. They find out if they've ever done any drugs. They find out where they live, where they work, get whatever negative thing that someone is willing to say or write about them, and reduce that black person, that man who's living their life like everyone else is, like everyone else who is human, and has done both good things and bad, into an animal that wasn't trained well enough to comply with police, and was put down accordingly.