Factual error: After the scene where Frank Castle steals the bus to rescue the children, the mobsters are firing at the bus. Not one bullet pierces a hole in the bus. Who knew Public Transit was bullet proof?
Factual error: After The Punisher rescues Tommy from his cell, they run to get Kathy. When they find her, Tommy says "Come on outside to the bus Kathy". However, there is no way for Tommy to know that The Punisher had brought the bus. (00:54:20)
Answer: With how many criminals Frank has killed, many of them would definitely want revenge. By hiding in the sewers, there's less chance of him or his hideout being discovered. If Frank lived anywhere above ground, his enemies would be able to find him more quickly and kill him.