In case you haven't seen them, some of my previous answers on the food supply and urban labor are useful for understanding the situation here.

Cities in the ancient world (and the modern world as well) were more or less without exception maintained by their hinterlands. Cities collect a large number of people who are not working the land and therefore not providing for themselves, and thus require sustenance from outside the city walls. With modern methods of transport and agriculture it's trivially easy in the developed world to provide cities with their necessary food from rural areas or abroad, but this was not the case in antiquity. As a general rule cities could not outgrow the supply capabilities of their own hinterlands. Too many people needing to be fed is a recipe for death. The city of Rome was highly unusual in that, like Athens, its surrounding hinterland was not especially fertile. Latium and the ager Romanus are fertile enough, but not so fertile as to be able to feed a city of around a million (the minor, but very much non-trivial differences in population estimates don't matter that much for the purposes of reddit) along with the various rural towns, some of which were quite large. Transporting grain from further away in Italy was prohibitively expensive, since it would have to be done overland--sea transport of bulk goods was universally cheaper and faster in antiquity. As the city's population steadily grew the pressure on the hinterland grew so acute that from relatively early on the city of Rome was forced to import large amounts, and eventually almost all, of its grain. However, foreign grain shipments resulted in uncertain supply and a much greater danger of the food supply's being cut off, as it did, for example, shortly before Pompey's command against the pirates. Compounding the problem was the threat of private suppliers destabilizing the market. Trades in grain futures, the creation of artificial scarcity to drive up prices, and stockpiling of surplus grain to protect sellers from the losses incurred during famine could create artificial famine, on top of the threat of natural famine, and cause wild fluctuations in prices. The distribution of free grain (really rather modest under Augustus, and more the possession of a privileged minority than the plebs at large) and, more importantly, the regulation of grain prices and supply to the market were therefore of crucial importance to the city of Rome, whose food supply was stretched far beyond its own hinterland.

By contrast, none of the other great cities of the Roman world were pressed by similar problems. Carthage (by the middle of the Principate certainly the second-largest city in the west and possibly in the entire empire), Alexandria, and Antioch all had abundant, extremely fertile hinterlands. In the case of Alexandria, as with the rest of Egypt, the Nile even provided a convenient "highway" by which grain could be cheaply, speedily, and efficiently shipped from the fertile regions upstream, should parts of the immediate hinterland ever suffer from bad harvests. As a result, as Erdkamp demonstrates in The Grain Market in the Roman Empire, these cities had little need for foreign grain, and only imported a fairly small portion of their food. Freed from the necessity to rely on the not necessarily secure foreign grain trade such cities had little need for the extraordinary legal regulations on the market that were necessary to make sure the urban population in Rome itself was able to provide itself with food.