I believe that violent crime started declining long before the current gun/SYG fad. AFAIK, people who study the issue are mystified. They don't know why violent crime has declined as much as it has.

Here is an outstanding book on the subject: The Collapse of American Criminal Justice: William J. Stuntz: 9780674051751: Amazon.com: Books

That book is a real eye-opener. Professor Stuntz was a brilliant and decent man who died way too young. I was fortunate to have him as a professor for a couple of semesters in law school. He was the only professor I ever had who was younger than me. Great guy.