Exactly how did they choose Bala to be the colony's princess? The queen gave birth every 5 seconds, and everyone was labeled either a worker or a soldier. What separated Bala apart from the other babies? Was she the first born out of billions? Was it because she was the most attractive (as far as ants go)?
Ant queens are physically different from the other ants, which can't reproduce.
In real life, ants are born the same (and all female apart from the drones) but a few are fed a different diet that makes them become queens. Presumably at some point they decided they needed to cook up a new queen, and Bala was the first larva that was suitable.