Beyond the fact that they eventually did quit, the shuttle program was the public face of the US space program, and part of having more than one way to launch military stuff.
The military rapidly gave up on the Shuttle after the first accident. No shuttle was ever launched from the pad on the west coast at Vandenberg (plenty of F9s have been, though.)
They were but the cost of refurbishment was almost more than just building new boosters. Thats why SLS is currently using Shuttle Derived Boosters but not worrying with reuse.