Because he couldn't win the siege. He'd be unable to choose the time and terrain of the engagement, Rome would have had plenty of time to marshal forces to fight against him, and he had no siege engines or engineers, so he'd have had no real siege works. This is a very bad situation to be in while besieging, because you need to still pay your men, feed your men, and otherwise keep morale up and sickness at bay (camps are not necessarily clean places to live in, and protracted sieges were noticeably worse in that regard)--all while being ready for relief forces to come and break the siege. You'd have to entirely surround the city (or at least cut off all avenues of approach) which divides your forces up quite a bit. He couldn't afford to do that either.

He didn't have supply lines into Italy--he had to get it all locally, which meant sending soldiers further and further afield if he was to set up siege. That means losses, that means delays, etc. Or, he could keep his forces together and move from place to place, trying to break the will of Rome's Italian allies. That didn't really work either, especially once Rome stopped obliging him with battles to win.

He was never really in a position to seize Rome.