"Why did the police use deadly force?"
Because he threatened deadly force against police, and refused lawful orders. The police in general have the right mindset: "my life is more important than anyone else's."
It is not worth risking your life to save the life of an aggressor who is threatening to kill you and brandishing a weapon to that end. It is more important to end the threat to your life, than it is to save the life of someone else at the possible expense of your own life. Everyone should follow this mindset and I feel sorry for anyone so guilted into believing a violent person who wants to kill you has any more right to live than you yourself.
Tasers also have an insanely high fail rate, are single shot weapons (prone to missing), use very light projectiles (prone to missing via a gust of wind), some people simply resist them, and sometimes tasers simply don't even penetrate clothes to be effective.
Imagine if police tried to use a taser, it failed, and then after unsuccessfully subduing the aggressor with a knife, he goes and stabs one of the several onlookers to death. Then they would be screaming "why didn't the police shoot him and end the threat?"