The earlier interceptors were for ballistic missiles. They are traveling at hypersonic speeds but have high trajectories (so radar can see them earlier) and can't maneuver for significant parts of their flight (so they are easier to track and target).

FWIW they were cancelled because they didn't have a particularly good kill ratio and proliferation and MIRV meant you'd need a ton of them to prevent an attack landing (and doing so would involve a significant number of nuclear blasts pretty close to the targets anyway). Deterrence was more credible.