Well, in all fairness to my fellow federal workers in DC, the overwhelming majority of the street musicians that play at DC Metro stops are just bloody awful! (In my experience...) That Bell should have been noticed because he was the exception may be true, but expectations are pretty low.
I was amazed, for instance, when I was lucky to visit Munich a couple of years ago, at the level of musicianship of the many street players there. (mostly college student age) However, I think that the small, but constant, crowds that they drew were mostly tourists. This, also, would not be the case at L'Enfant Plaza.
There is a slightly better level of music in San Francisco and New York, and the best I've heard has been in Boston. But nothing in the US is like Munich, or Rome, or London, or ...
As for photography, I also know that both context and audience are important. Some people just want "pretty pictures", and no amount of "artistry" will be noticed.