Too much people were needed.
The Apollo program was built to land a man on the moon as quickly as possible, with a high budget and immature technology.
It was very well designed for its requirements. However it is not the right way to sustain humans in space long term. It cost far too much, in fixed costs and per launch.
If you look at old films on how piston powered airlines worked, the amount of staff needed was much larger than the amount of passengers and the trips were short. Hence by definition only few could afford long trips. It's just mathematics. Everybody can't be a lord with servants.
Only with modern jetliners the amount of work needed per seat mile dropped drastically, and travel became possible for the masses.
Something similar has to happen for space launch. We can't have a significant portion of the population working in rocket factories / refurbishment shops / launch control, so it must be developed and streamlined a lot. In practice this means reusable rockets with high flight rates and low maintenance. Further out, we need to extract resources from space to drastically drop the needed amount to launch from Earth's deep gravity well.
The whole Apollo architecture was a result of immature technology, short time table and large budgets. NASA should not try to repeat it, as the budget and the political drive are not there, and it didn't result in anything long lasting the last time either. Instead, focus should be on lowering cost of more modest missions at first. The Space Shuttle attempted this, but it was too ambitious and inflexible. After they built the mammoth, all the money went into feeding it, and there was no money for developing new things.
"“NASA is an organization that is dominated by fixed costs. In business terms everything is in the overhead,” he said. The committee found, with some effort, that the fixed cost of NASA’s human spaceflight program is $6–7 billion a year. “The bottom line is that they can’t afford to keep the doors open with they money they’ve got, let alone do anything with it.”"
-Jeff Greason, more at: