This news item quotes Elon Musk as saying
it would make sense for the government to rely on ULA's Delta 4 and the SpaceX Falcon 9, phasing out the venerable Atlas 5.
"You don't need both of those rocket families," he said. "I think it would make sense for the long-term security interests of the country to probably phase out the Atlas 5, which depends on the Russian engine, and have ULA operate the Delta family,
I understand that since then ULA have followed quite a different approach, that of keeping the Atlas 5 and the Delta 4 Heavy and just retiring the Delta 4 medium, which happens to use solid boosters not used on the heavy version.
Its very easy to get bogged down in corporate mud-slinging regarding the strategies of both ULA and SpaceX to engage with government clients. That said, it does seem sensible from an assurance point of view to keep the Delta 4 medium instead of the Atlas 5, given the latter's dependence on the RD-180 engine.
Does anyone know of any technical reasons, or technical and manufacturing/economic drivers that explain ULA's approach. E.g. the solid booster topic may be relevant? Please try to keep it to rationale that is
avoid dipping into speculation about their business strategy, tax payer value for money etc