The cold war ended, and there was no longer a need to prove that we could send something up into space and rain down fiery death upon anyone on the planet. Hence, no more space program.
That's not entirely true. Even after the end of the Cold War the manned spaceflight program in the US has received several billion dollars a year in funding. It's more than possible to operate a quite proficient manned exploration and colonization program with funding at that level. The problem is more that NASA and congress have been completely ineffectual at using resources effectively. The ISS was almost entirely built and launched after the end of the Cold War, and it's total cost was at least 50% greater than the entire Apollo program, for example.
