Because New York is New York and London is London. Cops in New York are armed. Cops in London aren't (bar C019). Hoods aren't scared of the police on the street in the UK because the cops aren't armed. At most they're going to get hit with a taser until C019 can arrive at the scene. Gangs in America have members numbering thousands and they're national (even international) organisations. Bloods, Crips, MS13, 18th Street, Aryan Brotherhood all of whom are tooled up better than the police.
The question then is should we arm all beat police in London as it works so well for the cops in New York? If you think the time wasted filling in forms is bad now, waiting until you see the form filling and enquires that'll occur every time an officer discharges a weapon.