I was finally able to track down the post where they explain it:
A little snippet, their explanation is quite complete.
"The game, editor and pipeline all use commercial middleware
. It is developed by Havok and several other companies.
The licensing agreement for the middleware allows us to use that code in specific products, on specific platforms.
If we want to release editor + pipeline, we need to license the middleware specifically for this. How much would that be? Perhaps $1M-$3M. I'm guessing wildly here.
Stripping out that middleware would seriously hamper the functionality especially of the pipeline. We use Havok Physics, for instance. Without Havok Physics, the pipeline wouldn't be able to convert any of the physics meshes. We also use Granny. Without Granny, the pipeline will not be able to convert any of the character animations. Etc.
Re-implementing the necessary functionality of the middleware ourselves ("let's make our own physics engine / let's plug in an open-source physics engine") would take literally man-years. Licensing is cheaper in pure $ cost and faster (it works now instead of by 2012).
The pipeline also uses some code that is under GPL. Given that we do not want to release the full source code for the editor + pipeline, we would need to replace the GPLed code with other implementations.
The GPLed code is less of a problem than the proprietary middleware."Edited by [\/]Paris - 2/21/11 at 12:32am