The Apollo program was motivated by politics; no matter that the Apollo program was conceived before Kennedy's speech, the commitment to make it happen was driven by the need to demonstrate an American lead over the then Soviet Union in the space race.
Before, during, and after Apollo, there was also talk about sending people to Mars. Apollo 11 achieved Kennedy's goal and asserted a U.S. lead in space. So what would have been the motivation at that time to send people to Mars rather than perform robotic exploration?
Was Mars "the next high ground" in case Apollo failed? Were there ideas of commercial development (resource extraction) driving the interest in going there? Is the absence of a political motive (such as drove Apollo) the reason human missions to Mars were not pursued at the time? What was the motive fueling the development of all the Mars mission concepts which were floated at the time?