Nixon not only didn't care about space, he saw the Apollo program as a
living legacy of John F Kennedy, who beat him to the Presidency in 1960. He and Agnew couldn't wait to get rid of that tribute to his hated
former rival, even if the man were already dead and gone. Nixon knew
that JFK would always be more popular than him - and in a few years,
he would find out just how much more popular.

It still galls me that Nixon's name and signature is on the Apollo LM
descent stage plaques and not Kennedy's, or even better Lyndon B.
Johnson's. LBJ knew and appreciated the value of space exploration,
even if a lot of it was for political and national gain. JFK admitted that
he didn't really care all that much about space, just so long as the USA
could do something big short of nuclear war to beat the Soviets in the
Cold War.

The second reason is that NASA feared a major accident during one of
the Apollo missions that would kill the crew. Apollo 13 scared them a lot
more than the administration would let on and they feared that an even
worse event would stall or shut down NASA for years. So between this
and budget reasons (thanks Vietnam War), Apollo 18, 19, and 20 were
not to be, to say nothing of any permanent lunar bases or colonies.

As for Mariner 4, while it is true that the images and atmosphere data
it returned (plus a lack of a detectable magnetic field to boot) did contribute to bringing down the nation's desire to send humans there
(plus Mariner 6 and 7 didn't do much to improve that image in 1969,
and by the time Mariner 9 got to Mars to really change our view of the
Red Planet, it was too late), a lot of other political factors were already
in place to delay our destiny beyond Low Earth Orbit.

Here's hoping that some private group or company will pick up the torch
someday.