None of us have seen all the evidence. In particular, we haven't seen the defenses or evidence the defendants have.
However, Fani Willis has seen it.
If someone wasn't charged there is one reason: There wasn't enough evidence to secure a conviction at trial. Put another way: The person had a solid defense.
Remember: we have only seen part of the evidence.
Actually, a second possible reason: Willis screwed up, but there is no reason to believe that.