Um....could it be an issue of logistics and security? That many balls, not enough of the O-man? And, didn't Biden go to the Heroes ball? And...isn't the "Commander-in-Chief" Ball one where enlisted active duty and reserve military peeps appear? Which he did go to....
From his logic, I'd say, out of all the different pies to keep one's finger in, it is best to give appearance to different varieties of balls, and one military ball is enough.
We could perhaps then argue why appearance at the Commander-in-Chief ball vs. Heroes ball, but, it's kinda self-explanatory in the names, I'd think.
Aside: not that the O-man isn't a 'hero' (yet), just to imagine him in tights....those chicken legs. *swoon*