Changes in the country have made the national government much bigger and more involved in people's lives. This, along with the fact that the US has become much more involved in world affairs, makes the Executive Office of the President (EOP) much more important than it used to be.
Three of the most important organizations in the EOP are the Office of Management and Budget (OMB), the Office of the US Trade Representative, and the National Security Council (NSC). Over the years, the US government has become much bigger and has come to do many more things. This makes OMB much more important because it has to advise the president about how much to budget for the various things the government does. The US has also become much more involved with the rest of the world, both through trade and in military terms. This makes the Trade Representative and the NSC more important.