A threat to gun ownership is equated to a threat to the freedoms fundamental to what makes the US 'the best country in the world'.
Any argument restricting gun ownership, no matter how logical or however great the weight of evidence, is up against this "the very DNA of the country" emotional position.
Gun availability has been said to be similar in the US to some other countries. Mental illness isn't limited to the US. The US' foundation is based on a war fought for freedom, using guns. That's the differentiator. (this, my apologies, papers over the requested nuances of mental illness drugs, the cost and availability of medical care, the individual and societal effects of uber-capitalism at all costs, the equating of any level of socialism to be full-on communism, the widening ideological divide and/or the decreasing tolerance for the existence of differing ideologies to one's own).
My cynical take is: Why would this incident make any difference if none of the previous incidents have? It's a harsh observation, but this is "US normal", and has been for a long time.