It's not as simple as that. Advances in trauma medicine played a major role in lowering murder rates.
I've seen disputes about how much (can look them up), but as the abstract notes, this was largely a mid-1970s thing, as advanced trauma treatment from the Vietnam War filtered into the US, although the process started in the last '50s: https://en.wikipedia.org/wiki/R_Adams_Cowley
Murder rates aren't the only form of crime going down. Rape, robbery, property crimes are all down massively since the peak in the 1970s. One speculated reason is the removal of lead.
It's not like it happens very often in the US either. I don't know anyone who's gotten mugged.
I looked up some international stats on robbery, but they don't differentiate between mugging of people on the street and robberies at businesses, banks, etc. The US has a rate about x3 higher than my country (Israel), but again, it doesn't tell you much.
Sure, but 3x a very low number is still a very low number. It's not like I think or worry about being robbed.
Thought personally, not living in the US, just the thought of being robbed is like science fiction to me. And it's not like I live in a great country, but things like this just almost never happen here. Gas stations and convenience stores get robbed sometimes, your apartment might be broken into, but someone on the street robbing you? I don't know a single person this had happened to.