I think it's more the fact that the whole program feels like it is being stitched together based on which existing technologies and contractors contribute to which congressional seats, rather than which technologies are really a good fit in the long term. As well as the fact that beyond a fairly nebulous manned astroid-capture mission, there doesn't seem to be any great plan or will to have a concrete goal for the booster in general. If Congress earmarked $50B over the next decade to put a research station on the Moon or Mars and insulated it from the year-to-year whims that always infect NASA's budget process it'd be one thing, but they aren't. They're trying to build a rocket and then hope two administrations from now it gets a mission funded.
On the technical side, any believe there's no place for solid motors on crewed flight anymore except to ensure campaign donations from Thiokol and United Space Boosters.
Second, while waiting for the new SSME derivative to get finalized and into production, they intend to fly the existing engine inventory. As one of the larger flown relics from the shuttle program, and with several dozen laying around, many of us would rather see them distributed to smaller museums that didn't get orbiters instead of splashed in the ocean. And as a result of the decision to use up the existing stock, the entire expendable stack is built around an engine that's was originally designed for reusability, with all the cost and engineering penalties that implies, and is ultimately too small for the job anyway. If you don't try to fly the existing SSME stock, something like a larger, more modern F1 derivative may start to make more sense, enabling a more powerful liquid first stage without having to bolt solids on the sides to get it off the pad.