I think it really boils down to American voters having lower standards and being resigned to the fact that these things happen. I don't think a single politician has lost re-election over it recently. The last time there was a significant voter backlash over crime in New York was when Giuliani was elected mayor in the 1990s, and crime was a lot worse back then.

I suspect that elected officials would feel significant pressure to act if this happened in London on a regular basis.