The stranger—"Silky" Bob—has returned to the neighborhood after all these years because of a promise he and his old friend Jimmy Wells once made to each other. Jimmy and Bob had grown up together in New York, but Bob decided to head out West to seek his fortune. On the night before Bob left, he and Jimmy vowed to each other they would meet up again in twenty years time on that exact day outside "Big Joe" Brady's restaurant in New York City. This was the place where the two close friends dined the night before Bob left New York. Bob would choose to make his fortune as a master-criminal. But unfortunately for him, his old pal Jimmy would opt for a career in law enforcement.

Why did the man return to the old neighborhood after twenty years?

The reason that the man in the doorway has come back to his old neighborhood after so many years is that he promised that he would.  He used to have a very good friend named Jimmy Wells.  The two of them were just like brothers.

When they went their separate ways, they promised that in 20 years they would come back and meet again at the restaurant that was at the address where the man is waiting.  So the man in the doorway, whose name is Bob, is waiting for Jimmy Wells to show up.