There were several reasons. You can browse the old posts to learn some of the details. Essentially, there has never been enough interest in MM9 to support a more serious repair and modding effort. Support from UbiSoft and LithTech is non-existent.
There has been much more support for modding MM6-8. They are classic examples of CRPGs of a certain era and will always have an appeal. MM9, otoh, although it still has that NWC charm, compares poorly to other members if the first generation of fully 3D CRPGs.