The most important reason for the proliferation of interest groups in the United States is the fact that the government is involved in so many areas of our lives.
Interest groups form so that they can lobby the government to take certain courses of action on various issues. The more that the government is involved in our lives, the more government policies affect us. When we are affected by government policies, we are more likely to form groups to try to influence those policies.
Because government is so big, we have a great need to influence it and we form many interest groups to do so.