At least that is what I think the network thought would happen.
According to Memory Alpha, here's the producers reasoning for starting with The Man Trap:
"The Man Trap" was the first Star Trek episode to air, on 8 September 1966. As Robert H. Justman and Herbert F. Solow recount in their book Inside Star Trek: The Real Story, the decision to broadcast before any other of the of the few completed episodes was, largely, a process of elimination. Although it had good special effects and demonstrated the series' intelligent approach to alien life forms, "The Corbomite Maneuver" was not chosen because its completion was delayed by the post-production process and virtually all of its action took place aboard the Enterprise. The latter drawback also weighed against "Charlie X", which was further deemed "too gentle" a tale because it dealt with the problems of an adolescent. "Mudd's Women" was out of the running because it was questionable to lead off the Star Trek franchise with a risqué story about selling women to miners in space. The second pilot, "Where No Man Has Gone Before", was considered too "expository" in terms of its background to be broadcast so early. Justman actually favored "The Naked Time" because he thought it would provide an ideal introduction to the different characters' personalities. "The Man Trap" won out because its straightforward action plot was not considered too exotic, it had the advantage of a monster to thrill the viewers, and it fulfilled the series' "strange new worlds" concept.