Morton Thiokol is a solid rocket company in the northwestern U.S. The Shuttle launched from the southwest U.S. The solid rockets, with tubes of propellant inside, had to be cut into sections to make the long journey. The joints were heavy (very bad for rockets), and a leaky joint destroyed the Challenger.
The horrible Air Force requirements made refurbishing very expensive and slow. Everything had been stripped of so much weight that it was too fragile and needed detailed inspection and repairs for each launch.
Morton Thiokol was crammed into the program to get Shuttle votes from their state's senators. The catch is that the crammin-crap-in process doomed the program. But if Morton Thiokol had threatened to cancel the program unless it was reinvented, they could have come out as a real rocket company supporting a fleet that launched every month. Honestly, the innovations SpaceX is coming out with could and should have been done by the Shuttle program, instead of the idiot business "leaders" chaining themselves to the worst possible ideas