It's not poverty.

Depends what you mean by that. Someone who suffers from a mental health issue (or really any other health issue) can suddenly become too poor to afford proper care.

Assault style weapons are expensive.

You people keep using these generic terms without ever explaining what they mean. You can get a semi-automatic handgun (which is responsible for the overwhelming majority of firearm deaths) for ~$200 or a bare-bones AR-15 (which the entire category of "rifles" is responsible for fewer deaths than literally just hands and feet) for ~$300.