Maybe they aren’t used anymore? Maybe they are dangerous? All sorts of reasons. No one really says.
Maybe they aren’t going to destroy them at all.
Maybe the ammo will be redistributed to regime friendly agencies.
Maybe the whole story is a lie.
Properly stored ammo doesn’t “go bad”. I have a case of .303 British of which I’ve run 400 rounds through my Enfield without a misfire and it was manufactured in 1918.