A thread titled Messiah level editor on the Shiny Developer's Corner messageboard offers word from Shiny Producer Stuart Roch on why there are currently no plans on releasing the Messiah level editing tools with the game:

Currently there are no plans to release our level editing tools to the public. While it's always cool to release the tools to the gaming community to let everyone have a try at creating the next greatest level, the editing tools are VERY complicated. As crazy as it sounds, it takes our experienced level artists/designers roughly 6-10 months to create an entire level from scratch when you consider both the geometry and textures. Releasing the tools also adds some overhead to the development of the project. Even if we had no plans to support the tools with the end users, we still have to put together some sort of support documents to get people started and make sure that the tools have a reasonable level of user friendliness.

Stuart Roch, Producer
Shiny Entertainment