In 1962 the very first commercial satellite powered by solar cells, the Telstar 1 was launched. It was state of the art, designed by Bell Labs, but to put it into context, this 77 kg satellite managed to generate around 15 W of power. As a point of comparison, that was not enough enough to power Apollo 11's radio, which used 20 W for long distance communication with Earth, from the moon.

The LEM, the lunar lander, has a power bus that maxes out at 4 kW, and while running on battery, 350 W. The LEM used that huge power bus to run a variety of energy hungry devices from space heaters to sensors and gimbals. The Command module could pull a similar amount from it's twin buses. Both battery banks were designed for redundancy, and with extra watt hours in the case of an emergency.

The reality is that even improved over the intervening 7 years from the Telstar, space-fairing solar cells were a relatively new technology without the compact size or wattage that fuel cells provided.

Now, this isn't to say solar cells weren't common in space based applications. They were. But since they didn't fit the needs of a manned mission, NASA needed to invest in power sources that would. Early Mercury missions were plagued with electrical faults due to difficulties with their cell batteries. Some failed entirely, some lost telemetry or flew over or under due to voltage regulation issues. During Project Gemini, the bridge between the early manned Mercury missions and what would become the Apollo moon missions, NASA invested heavily into fuel cells. By 1962 they had already spent nearly 9 million dollars on a contract with McDonnell and General Electric. The completed fuel cell would fly on Gemini 5 and set the stage for further use of the technology.

Apollo's fuel cells ran off of hydrogen and oxygen, two cryogenic gases that were already used in multiple areas of the Saturn V, from the upper stage J-2 engines, to life support. NASA engineers were familiar with working with them, and since you already needed to invest precious, precious weight on pumps, stirrers, and tanks to carry the crew's oxygen, might as well double up and also feed that oxygen into your power system.

Fuel cells were a familiar technology, with clear advantages in total power output and size.

You can read a wonderful series of technical summaries from the original Apollo documents, preserved by NASA here. Specifically, you'll want to look at the technical review documents pre and postflight.