1) The NYPD budget is now much heavier on WOT related issues and this leaves less $$ and fewer POs for quality of life policing.
2) Rudy's plan of zero tolerance led to thousands of arrests of criminals who had jumped bail, violated parole or were wanted on warrants for crimes more serious than the turnstile junmping that they were stopped for. This led to putting much of the criminal flotsam away.....but not forever. They are back, having served their time and the system can't send them up again until they commit another felony.
So you have to leave them out on the street for what? Two oe three days tops?
My granparents lived in Rego Park, Queens during the late 80's and during that time, several drug dealers and prostitutes set up shop around Elliott Ave. The residents were routinely harrassed and threatened. The dealers/prostitutes would disappear when the police were called and reappear after the police left. Then one night, a prostitute got shot w/ a low caliber round in the leg. A couple of nights later, another was shot in the leg. After the third sniper incident (unsolved), the problem seemed to disappear.