As far as you know. Barry might have insisted that she not be there or failed to speak up and insist that she should be. We don't know. But the event has not happened yet and there is ample time for France to correct the "mistake" if they so desired. The fact that the royals are going public with this means that she was purposely left out by "someone".
Bingo. The State Department didn’t, as usual in this admin., do their homework. Even though it is a French planned event, the U.S. State dep’t would normally correct a huge gaffe like this. Of course, all this bunch cares about is “the one.