Because even after the Social War, the allies weren't really integrated into the political system. Then you had the conspiracies, leading to the first triumvirate (a conspiracy between the leading men of Rome to consolidate power and rig elections), and eventually the republic's abolition. After decades of lies and more lies, the Romans and the Italians had enough of "the nation" and just wanted the emperor to rule over them like any other king.
Later Rome wasn't about cultural homogenity at all. Romanization of Gaul was not about making the Gauls into Romans out of some sense of racial affinity, but a way of introducing Roman technology and law so those tribes could be administered. A Gaul was still a Gaul, a Briton was still a Briton, but now they had baths and the elite Gauls learned philosophy. Those elites were still kept far apart from the Roman culture - cultural assimilation into a whole just wasn't the goal. The Empire was about getting everyone to accept Roman law and military power, rather than using that power to assimilate everyone into the Roman race. They were very multicultural, and they didn't get why some people like the Jews just doggedly refused to go along to get along. That was how it worked in Pagan civilization, how it worked the world over, and every big empire in that time was multicultural or at least had the goal of ruling over multiple cultures and eventually the world. There were definitely attitudes that people not from the area were barbarians or uncivilized, but this was not a racial or national consciousness, but a judgement of other cultures' technological advance and level of civilization. The land was just a place, not a "nation" in the modern sense. You can even see this in a lot of the world where the development of the nation-state didn't happen, or was hurried out of dire necessity, which leads to people having a very different conception of what "the nation" means. And even among Europeans, their conception of what their nations are varies, for example the way America constitutes itself is based on an ideology rather than any particular racial heritage or culture.