The biggest reason was that the Apollo program was able to return film from the surface of the Moon to Earth. Many of these photos weren't returned to Earth in film form. Instead they were developed onboard the spacecraft, scanned, transmitted via radio, and then recorded onto tape on the ground. Every step of that process incurs a decent bit of quality loss.

You can actually tell which probes had a reentry module because the photo quality is pretty stunning compared to the photos transmitted over radio, look at Zond 8 which returned film negatives to Earth.