The main issue for sending troops from the Carthaginian empire to Hannibal in Italy was control of the seas and ports. While Hannibal generally controlled portions of Central Italy, he never held a strong port until Tarentum, at which point it was too late and he couldn't hold more territory. In addition, the Romans controlled the seas, particularly around Sicily, so it would have been very risky to try a massive troop movement. Without control of the seas, the only way that Hannibal could receive reinforcements was overland, the same way he came in. However, Carthage had been steadily losing battles in Spain, so there wasn't a lot of reinforcements that could come. Hannibal's brother Hasdrubal did try to come over, making the same risky journey that Hannibal had made over the Alps, but this force was ultimately defeated at the battle of Metaurus River by the Romans. At this point, Hannibal became unable to hold onto his territory, and at the command of the Carthaginian government, returned home to ultimate defeat.