My guess would be that it’s because Boston is one of the most Catholic cities in the U.S., ranking 7th behind three Texas cities, two Louisiana cities, and Providence, R.I.–
http://www.catholic-hierarchy.org/country/scus3.html
–and the fact that Boston is an archdiocese, and the fact that Boston has more priests than the next closest archdiocese, Newark (9th).
The Boston archdiocese had 901 priests in 2001, to serve a Catholic population of 2,069,225.
http://www.rcab.org/info.html
The Newark archdiocese had 550 active priests in 2001, to serve a Catholic population of 1,319,558.
http://www.rcan.org/planning/facts2001.htm
So Boston has nearly twice as many priests as the next archdiocese. Twice as many priests, twice as much potential for abuse.
My guess would be that as this goes more mainstream, we’ll see the shit hit the fan in other places. It’s possible that after people have seen the issue raised over and over and over again on various talk shows, they may be more willing to come forward.
However, the three Texas cities–Laredo, Brownsville, and El Paso–are also border communities, and there may be a lot of people who were abused but who are never going to come forward since they have shady immigration histories, and the less official attention they attract, the better for them.