- Orion doesn't have the necessary docking adapter.

- 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.

> 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.

In a nutshell it comes down to two things. A. It's not ready to fly yet. And B. it's not really built for that purpose. Sort of like taking an RV to the corner store. You can do it but it's expensive, bulky, and over-built.