Back in my busking days I would always choose a spot that created the effect of a stage: something to frame the "musical event." Bell stood in a hallway, stacking the deck against himself immediately. Come to think about it, stopping in front of where he was playing might have been difficult given the traffic pattern. Then there were the extremely-live acoustics. People could probably hear him all over the station, so there was really no reason for people to stand and watch him in order to listen.
Also, his choice of repertoire was not the best. He chose the extremes of darkness and brightness for his experiment. A serious piece like the Bach Chaconne, especially when it is well-played on an excellent fiddle, can put passers-by off because of its intensity: it never lets up. D-minor is also, even for people who don't have perfect pitch, a key that is riddled with darkness, especially when played on the violin. The E-major Gavotte en Roudeau that he played had a great deal of bright "in your face" intensity (it always does when played well). In the wild acoustics of a metro station, that kind of intensity can actually get kind of annoying.
On my best street-playing days I felt myself and my companions at one with the rhythm of the movement of the people on the street. It was often a wonderful experience. Bell was clearly not responding to the people in the station, except for the person who stopped to listen. He was kind of performing (but he also ended up kind of sounding like he was practicing). Maybe if his concentration had been geared towards drawing people in his "movable audience" he might have had more success. Who knows?
I think that he might have gotten more people to stop with lighter solo music like the Paganini 24th Caprice, the Bach G-minor Fugue, the Locatelli Harmonic Labyrinth, the Tartini "Art of Bowing," or the Tarrega-Ricci "Recuerdos de la Alhambra." Perhaps he could have tried the last movement of the Mendelssohn Concerto, the last movement of the Beethoven Concerto, or a movement from a concerto by Paganini.
Unfortunately the experiment can't be duplicated now that Bell's busking moment has had major press. The metro station might get some visits from curious Washington Post readers hoping for a repeat performance, and that spot might be taken by a real busker now that it has been written about. I hope that person has a better audience than Joshua Bell got, and I hope that s/he makes sure to get a permit.
Related post: Playing Music on the Street