Which is a reason why two major political parties do not have a strong central organizing structure?

Federalism is the reason why two major political parties do not have a strong central organizing structure.

Federalism refers to the mixed or compound mode of government, combining a general government (the central or 'federal' government) with regional governments (provincial, state, cantonal, territorial or other sub-unit governments) in a single political system.

Leading examples of the federation or federal state include India, the United States, Brazil, Mexico, Russia, Germany, Canada, Switzerland, Argentina, and Australia.