[Laremere]

The Apollo program had a wartime budget and a wartime risk tolerance.

The US and Russia were at war, though they did not directly engage with each other due to the threat of mutual nuclear destruction. Along with various proxy wars, technological dominance in space was a key factor in this war. If one side gained enough advantage, they could potentially leverage it into using it to win a direct war. Another factor is that Kennedy's assassination protected the program from political pressure within the US.

Since then, other factors have turned the attention of the space program: The USSR fell apart and didn't pose much of a threat, reducing the budget to a fraction of the size. The Space Shuttle was designed to be the next big thing in space, as a reusable launch vehicle; it could only do low earth orbit and fell short of its goals. Focus shifted to science, and a lot of good science could be performed in low earth orbit; This has lead to, for example, the significant achievement of a continuous human presence in space since the year 2000. Finally, the accepted risk for Apollo was several times what is acceptable today. Even if we had all of the old hardware on the launchpad ready to go for another mission, NASA would never put an astronaut on it.