Why did Hannibal not Besiege and Conquer Rome after his Victory at Cannae.

After the battle of Cannae one would have thought that Hannibal had Rome on the ropes. It is believed that one in five Roman or allied men of military age died at Cannae, Trasimene or Trebia. The story goes that Hannibal’s cavalry chief, Maharbal offered to take his cavalry to Rome and besiege the city, [...]