> Orion doesn't have the necessary docking adapter.
NASA actually enforced a standard docking systems for all capsules: https://en.wikipedia.org/wiki/NASA_Docking_System . In theory it's androgynous so it might even be possible to dock capsules to each other.
Orion is extremely heavy (26 tons fully fueled) so it would need a Falcon Heavy or D4 Heavy. This would make it very expensive.
It's also politics: since the early 2010s SLS has been the only "supported" vehicle for Orion thus tying the two programs together.
- Orion costs well over $1 billion per capsule, plus almost another $2 billion for the SLS rocket to ride on. In total that's more than the $2.6B they paid SpaceX for NRE + 6 flights. (NRE on Orion was $20billion and SLS will be even more)
- The SLS rocket isn't ready yet.
- Boeing can't build more than one SLS per year.