Apparently pickpocketing is way down. The kids just don't want to learn the skills when you can steal someone's whole bank account online.
http://www.slate.com/id/2286010/
Far higher profit margin!
And yeah, in the areas mentioned, it's all about the volume of people. Tourists are both easy marks and provide cover for behavior that would seem suspicious on a street that wasn't thronged with people. My street is a tiny, quiet, residential street -- and it's located off of one of the busiest streets in London so you've got to really stand out to stand out among the residents, workers and tourists.
We were robbed in a 2-hour window between 6pm and 8pm on a weeknight back in October. The robbers left our flat via a window, rolling my big bright red suitcase and my pink and red striped grocery trolley down the street -- full of our electronics and my jewelry. I'm betting nobody batted an eye.