"It occupied a brief, unique niche in time when it could have happened," David Mikkelson, founder of urban legend-buster Snopes.com, said of the broadcast. "It sort of required a single, real-time news medium that wasn't fully developed."
With no other news sources available to turn to for immediate corroboration of the event, it's understandable how people could have been duped, especially if they didn't know it to be a drama, or if they didn't hear the program's introduction: "The Columbia Broadcasting System and its affiliated stations present Orson Welles and the Mercury Theatre on the Air in a radio play by Howard Koch suggested by the H. G. Wells novel 'The War of the Worlds.' "
Legend has it that Welles and his company, which he co-founded with actor John Houseman, drew on such iconic audio as President Franklin Roosevelt's "Fireside Chats" and the off-the-cuff radio reporting of Herb Morrison, who gave the famous eyewitness account of the Hindenburg crash, to imbue their play with realism.
Welles and company turned a famous work into a radio broadcast within a radio broadcast, and it was very convincingly done if you didn't hear the beginning of it.
In case you never heard it or perhaps in case you HAVE heard it:
In case you missed the PBS documentary of the story behind the broadcast, here is the link.