The market was enough to incentivize merchants to transport food. The transport of agrarian produce, both staples and cash crops, is a fundamental basis on which agrarian economies grow. But that means nothing in the larger economy if there are other factors at play. First of all, the city was, like all cities, parasitic on its hinterland, and like all large ancient cities rapidly outstripped its immediate hinterland's ability to produce sufficient food at competitive prices. Grain could be got at greater bulk from overseas for cheaper, faster, and with more safety. The latter is important--the long-distance overseas grain trade was extremely risky, even if vastly more profitable and ultimately more secure than trade by land routes. Gil Gambash has pointed out the reasons why most sea trade would have consisted of relatively short coastal hops, as we know it was. For one thing, sale to relatively close regions that were accessible by ship but too far for land travel was still extremely profitable, could be done more cheaply than the long-distance trade necessary to reach the city, and was much safer--travel, particularly in winter months (when emperors like Claudius, despite the danger to ships from storms during that time, provided special stipends for grain-haulers), was still risky by sea.

Most importantly, probably, is that the supply of grain to the city is not the same as the stability of the market. Private grain sellers maintained massive stockpiles of grain to protect themselves from bad harvests and interrupted supply. While regulations existed in the Principate that prevented sellers from manipulating the market supply to keep their prices up, they were not always wholly effective. Additionally, just because there was nothing preventing supply into the city did not necessarily mean that grain sellers would have preferred to bring grain in. Causing artificial shortage--as was extremely common in the Republic--was a well-attested way of hiking grain prices, and was often economically more profitable to private sellers than maintaining a steady supply of cheap grain. It is extremely important to realize that most grain shortages were artificial. Even when harvests in the grain-producing regions were poor there was still grain available from somewhere. The real economic throttle was price and the manipulation of these prices past the point at which they were affordable to urban day-laborers.

Lest I reinvent the wheel, you'll find the links--and more importantly, the books--listed here instructive.