They aren't using Falcon Heavy because FH isn't human rated. In any case, Starliner could get by on a Falcon 9 (in terms of required performance), but it would

They aren't using Falcon Heavy because FH isn't human rated. In any case, Starliner could get by on a Falcon 9 (in terms of required performance), but it would be a big chunk of $$$. The Starliner is a pretty wide capsule, which is why they need that skirt on the Atlas V. The skirt controls the aerodynamic transition from the fatter Starliner to the skinnier Atlas underneath. The F9 is a different diameter from the Atlas, but still narrower than the Starliner, so they'd probably need a different skirt design. The obvious other replacement vehicle is Vulcan, which is actually fatter than the Starliner. I have no idea what integration issues that'd entail. As for why Boeing went for Atlas? Well... SpaceX is their literal competitor. Assured access also implies being willing to take a hit on cost efficiency to ensure diversity of both crew capsule as well as launch vehicle.