They let the solid rocket industry atrophy. Part of the reason for having SRMs in the Shuttle was to indirectly fund the solid rocket industry so there would be one in case they needed to redesign the ballistic missiles. For a long time there was also a policy to have at least two solid rocket companies, Aerojet and Thiokol, by throwing some work at them. The Delta rockets used Thiokol GEM boosters, and the Atlas rockets used Aerojet boosters. Then they merged both space companies into ULA, and the Vulcan rocket chose Thiokol. Shuttle is gone, and SpaceX does not use solids. There is SLS which does use solids, but they are by Thiokol, and just a minor modification of the Shuttle solids, yet they took forever to design and make them. Today you have basically Thiokol as a monopoly and that is it.
In theory with modern know-how you could design higher performance solids. You have denser explosives like CL-20. But I kind of doubt that will happen. It would balloon the cost even more since the industry to produce CL-20 at that kind of scale in the US does not exist.
