US police receive large amounts of hand down military equipment and are routinely armed.
Some US citizens are routinely armed, so the potential for lethal harm is much greater in any scenario.
US domestic policy has encouraged the police to be at war (vs crime, drugs etc).
US domestic policy has encouraged a criminalization of large amounts of petty stuff (truancy / three strikes policies) which encourages this.
This all encourages a us vs them attitude in police forces, where the police are fighting a 'war' against citizens.
US police are highly trained to stop a criminal with force, and rarely focus on de-escalation.
US police are rarely funded and motivated to make a visible presence beyond arrest etc, and don't perform much of a community role afaict.
Almost any civilian action (e.g. filming police / protests) encourages the 'us vs them' attitude and encourages police to close ranks.