The circumstances under which both battles took place vary quite a bit. When the battle of Zama was fought, the war has been going on for 16 years. At that point both sides were exhausted, but Rome was in a better position. It's lands were free of invading armies (although some parts devastated), and it managed to conquer much more territory as well. Rome's manpower pool was much larger than that of Carthage. Even though Rome suffered horrendous losses in the opening battles of the war, it was able to replaces those loses.

Carthage on the other hand, except for the string of victories by Hannibal and a few in Spain, was on the defensive for most of the conflict. It's possessions in Spain were lost by this point (battle of Zama), and it had no where near the manpower resources that Rome had. With the loss of it's Spanish holdings, Carthage lost a huge source of income and it's mercenary and locally hired soldiers. It also faced revolts by it's Numidian allies, and some of it's Libyan cities. (not all of them though).

Rome's armies were on Carthage's land, a short distance away from the capital. With the loss of the battle, Carthage had no alternative. It didn't have the manpower to raise another army, nor the time, nor the funds. Even if Carthage did win the battle, it had no means to turn the tide of the war. By this point it had no navy to speak of. There was no hope of landing an army in Italy again to threaten Rome. On the other hand, Rome had quite a large navy, and was able to land troops almost anywhere, virtually unopposed.

If Rome's army was defeated at the battle of Zama, chances are they might have sent another. Or possibly negations would take place again to try and end the war. Basically, Rome had a huge strategic advantage compared to Carthage at this point in the war. Even a defeat at Zama for the Romans did not mean a defeat in the war or a change in their strategic position. Only a temporary setback, for Carthage this was their deciding battle.

Source: "Fall of Carthage, by Adrian Goldsworthy"

Edit: Also, Rome's way of fighting a war differed in the way most states at the time fought wars. Remember when Rome was faced with a threat before Carthage, Pyrrhus of Epirus. He managed to defeat Rome's armies and march on its capital, but the Roman's refused to even negotiate. Back then, this was out of the norm. Normally both sides ended a war through negotiation, leaving the respective states intact. Rome fought wars to the "end". Meaning when they went to war they expected the other side to be utterly defeated and be neutralized to where it could not posses a threat to Rome again. Rome also dealt with each belligerent individually, making peace on a "case by case" basis. Thus ensuring the defeated state would be loyal or answer only to Rome, instead of dealing with an alliance as a whole.

When Rome was faced with major defeats at then hand of Hannibal, it was just as determined not to give in, or negotiate. Hannibal also did not have the numbers to besiege Rome, nor the resources. He did not have in his army any siege equipment. The equipment can be built, but takes time. Rome still fielded several armies across its territory that it could re-call. So even if Hannibal did march on Rome after Cannae, he would have to commit to a long siege. All the while cut off from any sort of supply base and in short time be threatened by another Roman army. Hannibal could not afford to sit in one place very long either, due to the lack of supply lines. His army had to live of the land, meaning he had to keep moving. Once his army consumed local stocks, they had to move to find more. Another reason why after Cannae, it would have been very difficult and risky to commit to a siege of Rome.

This is the kind of situation where we have to pull the camera back and look at the broader picture. War is ill defined by the direct battles fought and rather the campaigns and victories that lead to these battles.

Hannibal's overall military strategy for the invasion of Italy was a bold gambit that I would not advocate since it took the element of control away from the invading army and gave the Roman allies a "vote" on his success. Principally speaking, Hannibal intended to muster an army of mercenaries and backed by Carthaginians, use the wealth of Spain for supply, cross the Alps, and wage a destructive war that caused Rome's key allies and clients to abandon her. With this new force and without the shield of the surrounding cities, Rome would be alone and forced to buckle.

Hannibal's strategy could have worked, but as previously mentioned he was running on a gamble that he could not control. You can't force allies to abandon their overlord, you can only create an environment and hope that they do. Despite his impressive victories Hannibal inflicted only superficial damage on the Romans, killing army after army and chasing Flavius across Italy, but what does this do to the Roman war machine?

In truth, very little. Nations, especially with the manpower excess of Rome, are not brought to the brink of destruction by the loss of their troops, but by what the troops were defending. What did Hannibal ever take of value from the Romans? He did not deny their ability to produce food, his weak navy could not deny the Roman their ability to practice commerce, he did not besiege and starve the city, he just kept getting into 1 v 1s with generals while Rome was allowed to sit back and lick their wounds.

The Roman strategy to defeat the Carthaginians, when Scipio was at last unleashed, was a far superior one because it identified the elements that Rome could control and was waged against the Carthaginian dominion as a whole. For a long time Rome fell into the trap of thinking it was a war against Hannibal, when in truth it was a war against Carthage.

So Scipio simply bypassed Hannibal. His war in Italy was only possible so long as supplies and men were coming from Spain, so he made that his mission. He utterly drove the Carthaginians out of Spain, demolished Hannibal's supply lines, established Roman control of the pivotal peninsula, and won the support of the Numidians against Carthage, giving them the chance to fight back on the cavalry front.

With Spain gone and Sicily under the control of the Romans, with the Carthaginian navy still in tatters after the First Punic War, the theater of war was utterly in Rome's favor. Let Hannibal continue to dance around Italy occasionally burning a town or village, his empire was crumbling around him and staying for too long would see him starve. Scipio's eye was on the prize of Carthage.

This necessitated Hannibal's recall to defend his home base, which was a battle in which he called upon his own city's Sacred Band to bolster his mercenary veterans. At this point there was nowhere left to retreat to. If Scipio won the battle it was an open march straight to Carthage, there was no fall back plan, there was no continuation for the Carthaginian government, there was no Carthago Nova to set up a government in exile.

Because Rome's war strategy was all inclusive, and successful, Scipio was allowed to dictate the terms of the final battle, and he had the manpower, the seapower, the time, and the strategic climate that would let him simply starve Carthage into submission if Hannibal did not beat him there.

Hannibal never had that luxury in Italy.

So in summary, it was not the men lost that made Zama so important, it was everything that led up to it, the successful wartime strategy of Scipio, the utter collapse of the Carthaginian war machine, and the rigid, failed plan of Hannibal that made Zama so important. It was the final nail in a successful war, not the sudden burst of battle like Cannae.

People often look at the Battle of Cannae from the wrong perspective. While it was an amazing tactical performance by Hannibal (and an incredible tactical blunder by the Varro and Paullus), that was not, really, the most incredible fact about it.

The most incredible thing about the Battle of Cannae was that despite losing 70,000+ men in a total disaster, the loss of a consul, total demoralization of the remaining armed forces, and complete disarray in Rome itself, the Romans recovered. You have to understand the scale of what Hannibal had done to the Romans up to this point -- not just at the Battle of Cannae, but the previous battles in the invasion of Italy. Within about 20 months, Hannibal had kicked Rome's ass so bad and so often, that Rome had lost about 20% of its entire male population over the age of 17.

Despite this, however, Hannibal was never able to strike a decisive blow against the main thing that would have really damaged Rome: its economy. Wars are not won by defeating armies, but by destroying the logistical capability of armies to continue to operate. What allows this is men and materiel, and what supplies men and materiel is a robust economy. As long as Rome had trade routes with its allies and its agricultural and mining interests, it was effectively undefeatable. Eventually, the Romans realized, however, that the same was not true of Hannibal. They realized that his supply came mostly from Spain, and that if they could cut that off, Hannibal would be without men and materiel he desperately needed to continue his assaults. They invaded Spain, cut it off, also cut off his Numidian allies in Africa, and that led to the end of the war. The Battle of Zama was just the last gasp, it did not, in and of itself, have any real significance.

Just as an addition to the discussion here, which is all quite good and which I agree with. I would comment on the cultural values of the Romans opposed to the Carthaginians. Hannibal represented a master tactician in the vein of his Hellenistic counterparts in Macedon, Ptolemaic Egypt, the Seleucid Empire, and the Kingdom of Pergamum. As Hannibal and the previous Italian invader, Pyrrhus, discovered, the Romans never really surrendered and were willing to absorb crushing losses until final victory was achieved. In a way, the Romans had a much more severe view on surrender and not fighting to the death. For example, the most martial Greek people, the Spartans, were willing to negotiate for the return of the Spartan hoplites that surrendered to the Athenians at the Battle of Sphacteria (425 BC). The troops that surrendered at Cannae were denied ransom and the troops that had retreated from the camp and fought their way out were exiled to Sicily and given the shameful food of barley. This on top of the Roman traditions of group killings of those who neglected their watch duty and the system of decimation. I would say this severity and utterly stubborn military culture was enough of a deterrent to the Italian cities who had been subdued to remain that way. Those that did revolt, ended up back where they had started (Capua).

This severe military culture clashed with one that was far more understanding of retreat and surrender. Polybius mentions that at the battle of Cynocephalae the Macedonian phalanx on Philip's left flank attempted to show surrender by raising their pikes in the air. The Romans did not understand this signal and butchered them.

While the Roman severe system brought it a ferocious willingness to fight, it demanded rigidity that injured it tactically time and time again. Each level of the manipular legion mirrored Roman society and the demands for each class to showcase battlefield prowess. Velites, the youngest, attempted to show their manliness/courage/virtus by engaging one on one with other skirmishers. Contrast this with the triarii, the oldest and most veteran warriors who had already proved their worth. Triarii were the last to fight because the young wanted the chance to show off their own skill and warrior ability. The Romans did not ford the River Trebia because the consul of the day was a fool, they did it because they wanted to fight and win glory for themselves compared to their neighbors and tentmates. This headlong courage and demand to fight in the same formation cost it dearly and demanded a man who had proved himself so greatly in Roman eyes that it could be changed. This man, Scipio Africanus, had done the greatest thing a Roman could do, save his father, and had convinced the men at the Cannae camp to fight their way out rather than surrender. In essence, he had shown such a high level of virtus he could dictate changes to the manipular formation in the form of his disciplina.