Likely because it's a complicated, possibly unsolvable issue.

• It seems like a somewhat uniquely American phenomenon. Random mass casualty incidents occur abroad, but usually are different (Muslim on muslim, muslim on Whites, Israeli glowie shit, etc).
• While RWers don't like to admit it because it's a third rail, the 2A in the US seems to at least be partly responsible for mass shooters. As in, arming the citizenry will inevitably come at the cost of also arming a few psychos as well.
• Reducing mass casualty events likely involves giving over even more civil liberties & accepting even more surveillance from a corrupt & malicious State
• Mental health is very left-coded & effeminate. It's not very effective at preventing violent crime nor is it very good at rehabilitating criminals.
• Mental health is also too broad a concept like racism or antisemitism. When most people are talking about mental health, they're referring to garden variety depression or anxiety in housewives or maybe treatment of stark raving mad homeless schizos on the street. But the forensic psychiatry of the criminally insane is a whole other kettle of fish. And it's not really any more effective than institutionalizing the bad eggs and drugging them into a docile stupor.
• Low value dysgenic young men have no champions. Women mock them as incels. The left tells them they are oppressors. The right tells them to kill themselves.