For the answer to the questions above, I think the most significant thing is probably manpower. The Romans had the pool of hundreds of thousands of small farmers to draw upon. The Carthaginians used mainly mercenaries and federate.The Roman Govt also seems to have taken the war more seriously, as a 'total war', and mobilized all their resources earlier and more effectively. Combined with their larger manpower this meant they were able to absorb defeats more easily and continue fighting. I have read a book which used the analogy of a disciplined roman 'communist' society fighting a disinterested Carthaginian 'capitalist' society. Romans have a strong government system which is so organized and amazing how they developed that advance system that time
I hope my answer helped you.