Money.
Because NASA is funded, almost entirely, by the american tax payers (yes, a few of them still pay tax) so they couldn't justify the expense, especially during the 70's and 80's oil crises.
Vietnam didn't help. People started to hate the administration and the money spent on that, as well as what seemed like pointless space missions.
I go to Cape Kennedy and Houston every year for various reasons and it amazes me that with our current level of technology... they still manage to have incredible blunders which don't get out to joe public.