Why have most of the large countries in the world adopted a federal form of government?

The separation of national issues from local issues is important in dealing with problems, but addressing them requires coordination to be effective.

State or provincial governments are better suited to dealing with government functions in their specific locales, while a national government is required to deal with international issues or those that affect the country as a whole. A certain degree of state autonomy ideally will reduce the bureacracy required by a national department or agency.