Why is pork shoulder called Boston butt?

Because it was originally made in Boston. A true Boston Butt (Pork Shoulder) is usually made in Boston.

Actually the name originated in the other regions outside of Boston pre-revolution. The less expensive cuts of pork were often packed and stored in "butts" (commonly known as casks or barrels). The way the shoulder was cut and packed in Boston led to the name. Today the name refers to the cut of meat (front shoulder) not how it is cured or prepared.