Wearing a flag pin became a thing following the 9/11 attacks. Everyone was wearing a little pin, or a red, white, and blue ribbon, but as time went one, people naturally quite wearing them.

The Congress and the President needed to get support for the the most recent Iraq War. A lot of politicians in general, but especially Conservatives, were still wearing the flag pins. Or started wearing them again. They were used as part of the "you're either with us or you're with the terrorists" rhetoric at the time. (See also: Freedom Fries.) It was a very partisan political attack the right was using to say see, they're not wearing flag pins, they must not love America like we love America, only true Americans wear the flag pin and true Americans support the war. All faulty logic--that worked very well on a lot of the American public.

That sentiment was carried into further elections, and the dog whistle politics against Obama's campaign. He doesn't wear a flag pin? He isn't a real American. He must be Muslim! (As if that even matters...)

That's how not wearing a pin is a partisan symbol to attack politicians like Clinton who aren't wearing the pin for whatever reason. When someone stops to think about it, it makes no sense, and there are Democrats who were the pin.

Now why Clinton doesn't wear the pin? I could make a dozen educated guesses...Maybe she feels, or wants to appear, above the fray of the petty dog whistle politics and rely on her actions speaking louder than jewelry.

If she did wear a pin, there would be critics from all sides calling her out for pandering.