Because preachers have never committed adultery
Here's something, though: preachers almost never justify adultery, even their own. When the truth comes out, they are shamefaced, they are usually deposed from their church calling, and they may very well repent and shed bitter tears over it because they know they were wrong. In the best cases, they ask forgiveness of those they have wronged, and try to get back on the right path with the help of god, or I would say, God.
Whereas --- and I don't have survey statisics on this of course, just what I've seen in life --- atheists, though possibly no more likely to transgress the moral code, are far more likely to say "Screw the moral code" --- to reject the very standard by which they were judged.
It's what I've observed amongst atheist University faculty. Amongst the atheist literati. And elsewhere.