Probably with the lighting and stuff it was cheaper and easier to just digitize it than it was to call for a reshoot.
Well, it’s not like they needed Driver for this. They could have just put any guy with a Kylo mask in front of the blue-screen, the fiddled with it digitally to save time. Making a Kylo 3D model seem like a waste of time to me, bu then again when you have a 200 mill. budget I guess you have to use all that money on something.