Mostly, it has to do with trying to cram everything into a very tight timeframe. A few points:
The Apollo 1 mission was intended to be a test of the command and service module in low earth orbit. Before they even made it to the launch, all three crewmembers died in a tragic fire. So everyone from NASA to Congress was really doing some soul-searching about whether the risks were worth it and how they could best continue.
On top of that, the lunar module was also plagued with all manner of design and production problems, and it was disrupting the entire timeline. NASA decided that the best idea was to send a mission without the lunar module, just to see if we could make it around the moon. Apollo 8 was another low-earth orbital test of the C&S module, and at the last second they decided to turn a low-orbit mission into a real to a trip to the moon. So a process that normally took 12-18 months had to be compressed into 4 months.
Author Robert Kurson put it this way: “Planned, trained for, and executed in four months time… There were all kinds of other risks and the idea that we weren’t ready to do it yet. NASA had to build software very quickly and calculate the trajectories. Everything had to be rushed and compressed into this very tiny timeframe.”
On top of this, the first unmanned launch of the Saturn V rocket went okay but the second unmanned launch (Apollo 6) was practically a disaster. I suppose it was good in the sense that it revealed the malfunctions, but there were no intermediate launches to make sure the bugs were worked out. So launch number two was a colossal mess, and launch number three had live human beings strapped to it on a trip to the moon. (Apollo 7 was a low earth orbit test of the command and service modules, but did not use the Saturn V rocket.)
Recall also: There was no lunar module on Apollo 8. This meant that (A) the crew had no backup engine by which they might propel themselves out of the moon’s orbit if the first engine failed, and (B) the crew had no backup cabin they could retreat to, as they did in Apollo 13. It was one of those times literally everything had to go right on the first try, while simultaneously planning and building everything in a rush.
I recall one of the Apollo 11 crew (I think it was Neil Armstrong) made a remark -and I’m paraphrasing here - where he pointed out: Almost everything Apollo 11 did had been done before. Apollo 8 orbited the moon. Apollo 9 tested the C&S module and the lunar module together. Apollo 10 orbited the moon, detached the lunar module, and descended to a lower lunar orbit. The only thing Apollo 10 did not do was actually land on the moon. So by the time Apollo 11 came along, most of these things had already been tried and tested and cleared. The Apollo 11 crew actually thought Apollo 8 was the riskier mission, because Apollo 8 did not have those same advantages.