The general trend in American history has been for the states to become weaker and the federal government to become stronger.
In recent times, there have been many important changes that have helped to make this happen. Let us look at two.
The first was the Civil Rights Movement. This movement got the federal government involved in regulating such things as the workings of little restaurants and motels in various towns. This use of the commerce clause led to further expansion of federal powers in later years.
The second was the boom in the use of federal grants. These grants of money tended to come with strings attached, allowing the federal government to tell the states what to do. A major example of this in recent years is the No Child Left Behind law.