What arts are where tend to be based on who found success when they brought their art over from the Orient. A person starts up in an area, goes through a life time career of several hundred to thousand students, has twenty people go on to found schools who teach a thousand people each, and all of a sudden, a greater metro area has sixty to seventy schools which all teach the first guy's lineage.
Western PA has a large number of Tang Soo Do schools. A lot of them trace their lineage, in one fashion or another, to Chun Sik Kim - Not all of them are still with him, but he is responsible for the art flourishing here at some level.