Why isn't the engine's Case-Throat-Nozzle reusable in Virgin Galactic's SpaceShipTwo?

Question

I raed on https://www.smithsonianmag.com/smithsonian-institution/engine-powering-future-civilian-spaceflight-enters-collections-180971493/ (mirror):

RocketMotorTwo is a hybrid rocket motor that combines the simplicity of a solid propellant rocket engine with the controllability of a liquid propellant engine. The motor consists of two main parts—a large tank which stores the liquid oxidizer (nitrous oxide), and the cartridge which stores hydroxyl-terminated polybutadiene (HTPB) solid fuel. This combination, known as the Case/Throat/Nozzle (CTN), the guts of the engine, is the portion of RocketMotorTwo donated to the museum. It is one of the only non-reusable components of SpaceShipTwo.

Why isn't the engine's Case-Throat-Nozzle reusable in Virgin Galactic's SpaceShipTwo?

Answer 1

As your question states, a hybrid motor is basically a solid motor with oxidizer piped into it. For the reasons Burt Rutan chose a hybrid motor in the first place, see Why did Virgin Galactic decide to use a hybrid engine vs liquid on their space ship?

It's the solid part of the motor that makes reuse difficult. Reusing a solid motor case involves so much cleaning and refurbishing that it can be pretty much a wash as to whether it's just cheaper to make a new one - as the shuttle program found out and as discussed here: Why isn't someone building a fully reusable solid fuel rocket?