Ignorance of the climate in which the original mission happened, and ignorance of its actual goals.
The first: people at the time were mixed on the mission. It's a waste of money. Clearly it is; very little can be gained by sending bags of mostly water plus their support systems into orbit. Is it cool to do? Hell yeah it is. But just because it's cool doesn't mean it's worthwhile.
The second: an F-U to the Soviets. How do you win a war? With lots of money. Both sides spent ludicrous amounts of money on showing up the other. An all-out war involves traditional fighting: troops on the ground, air support, etc. In a cold war, you just don't send out troops. Nothing else is different. Just as much gets spent on intel, on developing weapons and defenses (even if they're unlikely to be needed), on securing geopolitical borders, allies and enemies, offenses and defenses -- just as much as ever.
Is
that a waste of money? Well, of course, but so is everything. There doesn't seem to be an obvious way to restructure the power dynamic in a way that doesn't involve wasting huge piles of money. It is the piles of money which supply the power dynamic in the first place; the best we can hope for, seems to be a dynamic where the money is spread evenly about. If we tried to constrict the money flow, we'd get a sausage effect and end up concentrating it elsewhere.
We might ask: was the space program, among others, a worthwhile show of power relative to other approaches? Would it have been better spent, say, strengthening and expanding our allies? Or more directly destabilizing our foes? Who knows. Even if we knew, hindsight is 20/20...
We might ask a completely different question.
What large project could we do today, which is likely to be looked back upon with rose-tinted glasses, by the next generation? Is that a loaded question? Well, of course, but so was the original question, and one must understand why, in order to construct a solution to it.
Tim