Hillary wasn't charged for the same reason Canadian senator Mike Duffy and several other senators were charged, and NOT convicted. Different pathways, but same result, the law simply wasn't specific enough to win a conviction.
Now we can play conspiracy theory as to the construct of those laws, but I hold that in both cases NO ONE would be stupid enough to try and sway a high-profile case such as this. My personal experience, in 25 years plus covering government as a journalist "hiding" that kind of complex move hasn't been possible since Watergate. I have seen high level investigations, one against a provincial premier, and the first thing that happens is that everyone's sphincter muscles slam shut like prison doors. At the top, any elected official who would even suggest trying will have the press on his ass like rice on white.
Like Canada, I suspect these "behavior laws" for our 'leaders' to follow are a slip shod mess written by junior attorney's with no experience. Unless there has been a scandal, these laws get little attention from over-worked reporters, and zero interest by the viewer until **** hits proverbial fan.