He didn't have enough men and Rome refused to surrender, some how kept most of its allies on its side, and waged a war of attrition.

Cannae almost broke Rome but it had a almost masochistic way of bouncing back from defeats in those days. I mean, they raised a new army of the same size and split it into some smaller armies so Hannibal couldn't seek decisive engagements anymore.

I mean, these are the same people that subdued Iberia by throwing man power at it for 100 years.