His army was numerically insufficient to undertake such a siege, and was destitute of the machines for battering the walls.

Rome was still defended by the city legions, besides which every man capable of bearing arms was a soldier.

The bitter hostility of the Latins would have rendered it difficult in the extreme for the army to have obtained provisions while carrying on the siege, while in its rear, waiting for an opportunity to attack, would have lain the army of Servilius, thirty thousand strong, and growing daily more numerous as the friends and allies of Rome flocked to its banners.
Hannibal saw that to undertake such an enterprise at present would be ruin.

His course was clear.

He had to beat the armies which Rome could put into the field; to shake the confidence of the Italian tribes in the power of Rome; to subsist his army upon their territories, and so gradually to detach them from their alliance with Rome.