@immortal-d it's not about priority. You'd need to set your AA to completely ignore everything except mercys and even then it'll be RNG whether they point in roughly the right direction, because the mercy just splits so far away from the target and turret rotation + projectile flight time just take too long.
making the bomb not (fully) tracking and making the split projectiles be targetable by AA might work to make it a more interactive unit and could have been an interesting change too, but whoever found the time to do it decided to take this way, so that's how it is now.