Because USSR's N1 program that was meant to be a rocket that competed with the Saturn V didn't get any traction. When its chief designer Sergei Korolev died in 1966 it was very demoralizing for the program. The N1 program was already few years behind the Saturn V development and very underfunded before Korolev even died. Then to nail the coffin, the N1 failed all 4 of its test launches. It was clear USSR could no longer compete and that America had won the race to space. American legislators recognized this and started dropping NASA funding back down, since after all, going to the moon was INCREDIBLY expensive and (and it pains me to even admit this) public support was already becoming dull.