Because we stopped. We no longer have the facilities and tooling. While we have the plans for the Saturn V (the only rocket to take people out of low Earth orbit), the manufacturing processes for some of the parts have been lost to time. To recreate a Saturn V, we'd need to figure out how to make them again with sufficient quality.
From there, there are two reasons why we have lost the capability - one is that, due to the perceived high costs in both money and lives, NASA has been forced to become extremely risk-averse. If NASA missions fail, it becomes a point of politics, not of science and engineering. The other is that there is no such thing as a "general purpose rocket", and there are no general purpose rocket parts. In order to design a launch system, you have to define the requirements that system must meet. And, again due to politics, NASA has not been allowed to settle on what missions their next heavy launch vehicle is supposed to serve.
Between the risk aversion, and not knowing what the mission is, development on a new system is extremely slow. Compared to, say, Space X, which has no issues if their test vehicles blow up, so long as the result is a reliable flight. They get to revel in and learn from mistakes in a way NASA cannot.
