Why are US political parties so decentralised?

US parties are decentralised for a number of reasons. Firstly, America is a federal democracy, meaning that the power of the government is not centralised into one body - instead it is divided between the states - the states having as much power and authority as the central government. The decision for America to be a federal country exists because in the years after the War of Independence it was found that the states couldn't …

…Party Conventions. There is a meeting of the national convention every four years, in order to select the party's candidates. The convention also determines party policy through a platform in which a stand is taken on the issues of the day. It also adopts the rules of the party. The national committee authorizes the selection of delegates to the convention. These delegates may be either appointed by state party organization or through the primary process.