I think their use of 3rd party libraries could prevent them from releasing it as open source without rewriting a bit. This even happened to the original DOOM 1 where they used a sound library so didn't release the original DOS source code. Rather they released the Linux port which used a different sound engine that had different features/bugs.

Looking at the start-up of Doom Dark Ages (with new expansion today), they list Havok, Oodle, Bink, and SpeedTree. According to Havoc's website, that already starts at 50k$ alone. Oodle/Bink don't list prices.