US primaries - could somebody explain?

[TinCow]

Non-Americans need to keep in mind that a lot of this complexity is due to the history of the United States, particularly the fact that in the beginning it was a collection of States being governed by mutual consent, rather than a single government with subordinate regions for managerial efficiency. The various States have many, many rights to determine how they run their own affairs, independant of the Federal Government or the majority of national opinion. Since local election legislation is within the States' jurisdiction, each State develops its own system, resulting in many, many variations. Over time, they are becoming more homogeneous, but it's a slow process.

The best way to think of it is like a theoretical EU in 200 years time. The EU was founded on a system which gives individual nations vast amounts of power over the central EU government. If, over time, the EU central government grows stronger, the individual nations will be required to relinquish some of their powers. This will draw them all closer together and may eventually make the entire EU itself seem like a single country. However, because the EU was originally founded with a strong emphasis on the powers of individual nations to determine their own affairs, even 200 years down the road, there will be many differences in the way law and governance are enacted in different nations within the EU.