One dynamic that may be at play: Designing a new engine is a gamble. For a company who already makes money with existing designs, there may be little upside on a small-volume new product. This is likely the reason the "Big 3" passed on designing an engine they don't have an off-the-shelf option for.

Boom would probably be the primary beneficiary of such an engine existing, so it isn't all that surprising that they will have to fund the development. The second supersonic airframe that can use their engine would be the one to turn that R&D profitable for Boom (and ease the introduction of more supersonic airframe options).