The problem here is that the protestors are against what they perceive as excessive use of force. Sending in the national guard would escalate the situation further. One of the reasons these protests persist is that they elicit from the police much of the behavior they detest. The protests are self-fueling.
David Dorn, a black ex-police officer, and 14 other people died due to the unhandled looting.
If they would have sent National Guard in to protect storefronts those deaths would have never happened.
Those peaceful protesters deserve protection from the looters as well btw.