Thiokol is now part of ATK, and Constellation was their baby. They destroyed the Shuttle SRB tools before requirement in order to push the Ares one forward. Unfortunately, ATK has Senator Orrin Hatch in their pocket, and he's a stubborn old mule.
There are designs which do away with the SRB, I'm helping out with one myself, but the political lobbying power of ATK is insane. Their SRB production line is a full 0.5% of Utah's GDP, and Senator Hatch is not one to let that go. With the ICBM's gone, they would loose their large segmented solid business entirely. Aerojet, the other SRB maker, saw this coming in the 1960's, and migrated everything to non-segmented solids (they also have the record for the worlds most powerful engine, a solid rocket engine called the AJ-260-2 if you ever want to look that up). There are still solid motors, but nobody else in the US uses, or makes, large segmented solids. Only France also makes large segmented solids, for their Ariene V launcher and their own ICBM's are rumored to use the technology, but nothing is known for certain. And the Ariene is due to have those removed for the Ariene VI and replaced with flyback, liquid boosters.