One of the most important factors was the rise of centralized government. As nation-states emerged, they began to consolidate power and authority at the center, weakening the power of local and regional governments. This process was often accelerated by wars, which led to the concentration of power in the hands of the central government in order to effectively mobilize resources and defend the country.
Another factor that contributed to the decline of regional autonomy was the development of modern transportation and communication technologies. These technologies made it easier for central governments to communicate with and control far-flung regions, reducing the need for local autonomy.
Finally, the rise of mass media and the spread of nationalism contributed to the decline of regional autonomy. Mass media helped to create a sense of national identity and solidarity, while nationalism encouraged people to identify with their nation rather than their region.
As a result of these factors, regions have generally had less autonomy in the modern era than they did in the past. However, there are still some regions that enjoy a significant degree of autonomy, such as the autonomous communities of Spain and the constituent countries of the United Kingdom.