As the growth of spending mandated by law has grown in the United States, it has put pressure on "discretionary" spending, which Congress authorizes on a yearly basis.
Discretionary spending has shrunk as a total of the federal budget over the years, as shown in this animation, even as total amounts have grown. This squeezes vital discretionary spending programs, such as space exploration and scientific research.
Charts and data are from the American Association for the Advancement of Science. They have an excellent section devoted to anaylsis of science and research funding the United States.