on occasion the state ,during the late republic period at least, just forgave all personal debt

They didn't. Debt-cancellation, or tabulae novae, was frequently proposed or demanded, but typically failed. Off the top of my head not one single successful attempt at tabulae novae exists in the Republic. At least, not in Italy: entire cities, provinces, and sometimes even kingdoms went into debt, and those essentially foreign debts are something of a world of their own. The definitive work on Roman debt is Frederiksen's 1966 article "Caesar, Cicero and the Problem of Debt."

Debt loomed large in the Republic, and in the ancient world generally. We like to imagine that debt affected only the poor, especially given the prominence of tribunes in proposing tabulae novae and the harsh punishments on defaulting debtors even after the abolition of nexum by the Lex Poetilia in 326, but the ancient economy ran on debt. Debt is effectively credit, it means that you have an economy that is able to handle transactions in the future, which itself means that there's a highly developed trade economy. Debt existed at all levels--indeed, I should note that the oft-repeated line that Caesar was supported by debtors and Pompey by creditors is total nonsense, and was firmly disproven by Shackleton-Bailey and Brunt way back in the early 60s, when they found lots of creditors and debtors alike in both camps, with no apparent pattern. Its repetition is inexcusable. Farmers, who in a pre-Green Revolution world lived very much at the mercy of nature, used credit all the time, to purchase their land and to offset the disaster of a bad harvest. Urban dwellers likewise used credit frequently in lieu of cash, which was not always on hand--the conjoined demands of tabulae novae and rent-relief show how credit was used for all sorts of transactions. But credit was also found among--indeed, was endemic to!--the senatorial and equestrian classes. Every senator was in debt, to other senators, to professional creditors, potentially even to their own clients. Debt and credit worked at various levels of formality, but credit was so important to the ancient economy, since it freed consumers from the need to transport huge amounts of cash around, that it formed its own transactional economy, wherein the IOUs of debtors (especially wealthy friends) could be exchanged. The importance of debt has not gone away, it's like having a grocer's tab or even (in the very short term) a bar tab.

The senatorial lifestyle was extremely expensive, and calls for debt relief originate frequently among senators as well as the people. Stereotypically, massive debt was incurred by senators of particularly depraved character, but in reality everyone was affected by it. Cicero's consular colleague Antonius was brought up for defaulting on his debt, and Cicero joked that he had borrowed so much money to afford his Palatine house that he would join any revolution that presented itself. The cost of elections, of holding magistracies (which typically found most of their funding from the private funds of the magistrate), and of keeping up the necessary amount of landed (not cash) wealth to remain in the senatorial circle was not easy. Paranoia about debt, however, was common. Even after the abolition of nexum, debt laws heavily favored creditors, and we repeatedly find the general opinion expressed that actions like tabulae novae would destroy the economy. It's not too hard to see why, even if many of the fears were misplaced. The economy ran on creditors, and needed them to function at all. At the same time, the excessive freedom granted to creditors and the uneven burden placed on debtors could often have the opposite effect on the economy. Cicero talks about debtors who are afraid to declare insolvency because of the harsh penalties associated with it. Their solution is to borrow to pay off the creditors, which causes them to fall into a never-ending cycle of debt (the problem was particularly well known to the senatorial class). Similarly, creditors effectively controlled the cash flow of the state. Political and economic crises often led creditors to hoard their cash (in much the same way that grain sellers could, before the cura annonae, hoard their grain during famines, making the problem worse, or sometimes intentionally created shortages to drive up prices) to ride out the storm. That alone would lead to a shortage of cash, but the problem was often made worse when creditors simultaneously called in all their debts in order to build up that cash reserve. When no cash is flowing and creditors are calling in debts en masse the economy collapses. But this does not only show the measures by which creditors often went to far. Credit itself was so central to the developed ancient economy (as it is now) that general debt relief was simply impossible. Imagine what would happen to the modern economy if all personal debt were suddenly abolished? How would that work exactly? At the very least it would be a gigantic social upheaval. The same was true for the Roman world, and explains easily why tabulae novae, though called for frequently, never actually happened, and was sometimes met with military force.

The problem of debt and credit could not, as Frederiksen (whom I've basically summarized above) makes clear, have been solved by tabulae novae in any case, and was not. The basic problem was that the creditors were afforded too much trust and debtors were subject to too much punishment in the event that they could not pay. This resulted in a delicate system that, while in practice fairly stable (if exploitative), could be very easily destabilized, with huge results on the economy, to the point that during some debt crises the state's coin supply ran short. The problem was mostly solved by the time of the Principate, though debt crises, usually of a slightly different nature, pop up from time to time. Caesar in 49, when a great panic was expected because of the beginning of a civil war (the same had happened in 63 and earlier), quickly put into place a number of temporary and permanent measures to encourage the flow of cash. Well short of tabulae novae--indeed, Caesar was heavily opposed to tabulae novae, and responded so strongly against several opportunistic attempts by supporters to get debts cancelled that Antony famously shut down Dolabella's debt proposal down violently, to Caesar's ire--, Caesar's activities allowed debtors a certain amount of temporary leeway, and combined with the confiscation of the treasury managed to keep cash flowing in Italy. Moreover, the establishment of new means by which debtors could default on loans without suffering total disgrace (a legal status particularly damaging to senators) and complete liquidation of their property, allowed for a more stable system of credit going into the Principate. In particular, the creation of the procedure of cessio bonorum, by which a debtor could go to the praetor and declare insolvency, ceding his property while retaining enough to survive (even if the total was not equal to the debt), allowed a stable procedure by which debtors could declare insolvency without fear of total poverty or disgrace (infamia), and still granted creditors enough benefit to allow the economy to continue moving even among insolvent debtors. The procedure, which is more like the old procedure of proscriptio bonorum (public auction of all the debtor's property, to be divided by the creditors, typically with the addition of the legal status of infamia) than tabulae novae, formed the basis of debt law in the Principate and into the Middle Ages.