When people focus on what would benefit their region or section of the country, rather than the nation as a whole it is called what?

Sectionalism

Sectionalism divides a country's loyalties according to the local interests of each section or region.
As an example, you could look at how different sections of the country might react to protective tariffs.  In the sectionalism that occurred in the Civil War era, the South was against tariffs because they relied on the import of British goods.  The North supported tariffs to protect their homegrown industries.
In the present moment in history, the US president recently announced a plan to impose heavy tariffs on steel imports.  That sort of policy is supported in areas like Pennsylvania where US steel production is centered.  But such tariffs are opposed in other sections of the country and other sectors of industry that rely on affordable steel in making their products -- such as automobiles.