I guess they couldn't hang a red lantern around the target. In truth, most anti-missile tests are highly suspect. Back in the '80s, the Pentagon even had a bomb to blowup the target vehicle in case of a miss. Washington junkies have been hooked on Star Wars illusions. I heard Edward Teller, the Dr. Strangelovian father of the H-bomb, expound his nutty idea for a nuclear explosion in space to trigger a laser beam.
Reality never dampens fantasies.
It survived declining defense budgets. It survived a fall-off of public interest so complete newspaper readers thought the program died. Well, politics surely. Newt Gingrich's Contract for America pushed missile defense. So Clinton and now Al Gore don't want to look soft on defense.