Basically they weren't originally planning on taking a photo of earth from the moon. The fact that they even had a camera was due to interest from a few astronauts in the earlier missions. The camera had its viewfinder stripped to save weight, so the astronauts couldnt see what they were filming. On that mission they were only planning to take photos of the moon's surface. And then they decided in the moment as the earthrise happened, that it needed to be photographed, and out of tons of shots they only got one clear image.

Maybe a little clickbaity but "It's highly imporbable that they got this shot" doesn't ring the same