Because a city so large needed a massive amount of land and the centralization of power to control what happens on that land to feed it. The annona was, among other things, the tax with which the Roman empire fed the city of Rome. There was a Prefect of the Annona whose sole job was to oversee the supply of grain to the city (and later, another Prefect of the Annona was established for Constantinople).

Because of the way it was conquered by Julius Caesar, and passed down to Augustus, the Province of Egypt had the odd position of being the personal possession of the Emperor, as opposed to a province in the way we would recognize it. Egypt, thanks to the Nile and its flooding, was one of, if not the greatest breadbaskets of the Roman Empire, and large portions of the grain grown in Egypt were shipped to Rome, where the Prefect of the Annona was in charge of getting it made into bread and handed out to the people of Rome. Any person who was a Roman Citizen in the city of Rome was allotted a certain amount of bread, though often if a citizen didn't need it, they would sell their allotment of bread to poor non-citizens. This is part of what Juvenal was mocking when he coined the term "bread and circuses" from this passage: " for the People who once upon a time handed out military command, high civil office, legions — everything, now restrains itself and anxiously hopes for just two things: bread and circuses".

EDIT: I'm rereading the paper I wrote on this in undergrad, and realized something I said above wasn't quite right- in the 1st Century AD, when OP was asking about, it was given out as grain, and only the rich could afford to bake it into bread. The poor ate it as oatmeal. In the 3rd century, Septimius Severus and Aurelian added olive oil and wine to the frumentatio, and Aurelian started giving the grain out prebaked into bread, which required the construction of massive bakeries in Ostia, Rome's port. It was important enough that to ensure things went smoothly, various emperors offered tax breaks and citizenship to the people involved in the process, the ships captains who transported it and the millers and bakers in Ostia.

Sources:

Garnsey, Peter. Cities, Peasants and Food in Classical Antiquity. Cambridge: Cambridge University Press, 1998.

Garnsey, Peter. Famine and Food Supply in the Graeco-Roman World. Cambridge: Cambridge University Press, 1988.

Sirks, Boudewijn. Food for Rome. Amsterdam: J. C. Gieben, 1991.

Bowman, Alan and Andrew Wilson. “Quantifying the Roman Economy: Integration, Growth, Decline?” in Quantifying the Roman Economy, edited by Alan Bowman and Andrew Wilson, 3-84. Oxford: Oxford University Press, 2009.