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 agree on certain things, such as trade and that in order for America to become a stable country power had to be given to each individual state so it could decide for itself how it wanted to organise certain aspects. The aspect of State representation is one of these aspects and this means that a State has a certain number of Representatives based on how big the population of a state is, and a set number of Senators.
Because each State has Congressmen, their ideas will differ from another state's because of the fact that the US is a massive country with a diverse population. This offers one reason why the US parties are so decentralised. The parties also have loose ideology and only meet occasionally every 4 years - mainly in the run-up to the Presidential election.
However, the main reason lies in the Parties' organisation. In America, parties are usually organised like pyramids having millions of party members and thousands of local party officials at the bottom; and national heads at the top. Elections and offices are organized on a national-state-local basis and parties are organized in a similar manner.
The first stage is Party workers selecting people who will compete for office, managing and financing campaigns, and even coming forward with policies and positions that appeal to the constituents. The next level...