Space Shuttle came about because Saturn was not economically sustainable. NASA was getting those massive funds from the Gov for the Moon mission, and after the goal was achieved those funds stopped flowing, and NASA had no way to maintain those expensive rockets.

So NASA came up with a reusable spacecraft design, the Space Shuttle. But DoD wanted the shuttle to have a certain payload capacity/features for their use cases and that forced NASA to change their original design, which also made it a maintenance nightmare.

In hindsight, Space Shuttle would have achieved its mission of being a cheap reusable spacecraft, if NASA were able to use its original design.