Apparently there were some nervous lawyers involved:
The source code for Doom 3 has been promised for a while now, but got a final hold up in the form of nervous lawyers at Zenimax. Carmack’s Reverse was the sticking point, as Creative Labs owns a patent relating to it. That could have meant months of delays through talks and agreements, but Carmack decided to solve the problem by rewriting some of the code.
http://zerowing.idsoftware.com/linux/doom/
