It's been a while since I've looked into it, but the official story is that they lost the original footage (I don't remember if it was recorded over or just thrown out). One of the space walk photos is clearly a mirrored version of an underwater training exercise picture that was also publicly released.

Basically, it's beyond belief to me that the original moon landing footage was accidentally lost. I also find it beyond belief that they would only fake one photograph, if they were going to fake any.

The theory that makes the most sense to me is that they knew photographic evidence would be destroyed by radiation (and maybe that they couldn't live stream video from that distance), but they also knew how badly the public wanted to see it, so they fabricated all the promotional materials.