Considering that there is plenty of oxygen in the air at the altitudes that the SRBs flew to (<50 KM), why did the STS SRBs use solid propellant. Specifically, why use a system that must carry its own oxidizer as opposed to a jet engine?
My guess is that solid fuel was used due to the amazing thrust those things provided (12,000 kN). However, a hypothetical jet engine would be able to throttle or even shut down, increasing crew safety. Unlike the first stages of other launch systems, the SRBs even had an exposed surface in the direction of flight, providing a convenient air intake.
I suppose also that the salt-water recovery might also be an issue with jet engines.