[jccooper]

Well, Congress specified it to use SLS, since they were looking for extra payloads. And then uncoupled it from SLS when it became clear it couldn't even handle one extra payload.

NASA, wisely, always benchmarked the mission on Falcon Heavy, and bailed from SLS as soon as they were allowed to.

Clipper on SLS was more of a "wouldn't it be neat" scenario than the intended mission design.