30 years of cuts to social safety nets, and life becoming affordable will do that. When as a country we prefer funding more policing, and less on affordable housing, healthcare, and education, we end up with a crisis like this. The fact that people act surprised is mind boggling. People have been warning about this for decades. I fear that this is only the start of a worsening trend.
This is demonstrably not true. Look at new york. They bring in broken window policing and do it passably well and things turn around in a decade. They then stop enforcing the law and move to a more laid back, defund the police style and things turn back towards trending to crap again. The other stuff you mentioned matters but the biggest thing is effective policing of all crime.