What book are you reading? Some writers care more or less about this issue. I'm curious what you're looking at.

So here's the thing: Italian citizenship status changed in time, as you might suspect. Rome in the Early Republic period organized the Italian peninsula through treaties. There were various categories of relationships that imposed different rights and obligations on Rome's neighbors, with the most important being "Latin." At first this was a recognition of cultural similarity, but later it became a legal status. It included the right to marry a Roman and to become a Roman citizen if you moved to Rome.

This treaty system worked well for Rome for a while, but following the Punic Wars it became increasingly problematic. Rome generally required its allies to provide soldiers on the basis of one legion of Roman troops to one of allied troops. As Rome began collecting overseas provinces, its military commitments grew, and it relied increasingly on these allied troops for support. Scholars differ about exactly where Italian discontent with this system arose: some think it was the dearth of men for farming that upset rich Italian land owners, others think it was Italian businessmen who found themselves funding Roman imperial adventures, and others think it was the soldiers on these long deployments who, while abroad, developed a sense of camaraderie with Roman soldiers, which, when they returned to Italy, they discovered wasn't shared by Romans at home.

For one of these reasons, or perhaps for all of them, Italians found their position as inferior allies of Rome unsatisfactory, and in 90 BCE a bunch of them revolted from Rome. They founded their own capital at Corfinium, which they named Italia; they formed their own senate; they issued their own coinage with "Italia" stamped on it. It's not clear what their goals were: they might be fighting for the Roman citizenship, they might be fighting to establish a new equilibrium with Rome as a lesser partner, they might be fighting to destroy Rome as a regional power. In my opinion, probably all of those motives were in action for different Italians, but we don't have enough information to be sure.

After two years of very vicious fighting, Rome won that war. Part of the settlement was making all Italians Roman citizens. That policy of inclusion became a theme of Roman governance and as Rome continued into the Empire you will find grants of citizenship to various groups of people until the whole free population of the empire becomes citizens in... 212 CE?

We can get into the nuts and bolts of this situation if you tell me what you're looking at precisely, but I've given the overview, which should be a good starting point.