Tom,
Boston butts comes from high on the hog, above the shoulder blade, and has lots of juicy, marbled fat. Below the butt is the pork shoulder. This cut includes most of the hog's front leg quarter. Because the leg muscles work a lot more than the back, the meat is a little tougher here than the butt, thus requiring a bit more time to come out tender.
A shoulder cut with the shank -- or hock -- attached is called a picnic ham. This cut is cheaper than most because it requires less butchering and has more bone in it.
A picnic ham is what people on the East Coast uses for pulled pork. Barbecue restaurants that make a lot of pork will sometimes use this because it's cheaper than Boston butt -- but also because some prefer the flavor. The shank and leg, if cooked properly, is full of flavor and has good texture to the meat.
To answer your question of why, I think comes down to a regional thing and personal preference. Both can be used for pulled pork.