The linked site spells out the explanation.
The Pennsylvania Railroad's course through Philadelphia has always been inconvenient for passenger trains. For years all services between New York and destinations south or west of Philadelphia reversed directions at Broad Street Station, the original downtown station just south of the Suburban Station. The Broadway Limited and a few other long-distance trains bypassed the 30th Street Station which eliminated the reverse move for trains bound to Baltimore, Washington, and Dixie.
The Suburban Station was so-named as it was designed with the intention of serving the electrified commuter service to the Philadelphia suburbs. All such services were routed at Zoo to the north or Brill to the south onto tracks that crossed the mainline and called at the upper level of 30th Street before heading downtown.
Very few long distance trains used Suburban Station, although for a few years the Amtrak Philadelphia-Harrisburg services ended there.
