There's a bit on an answer on the modding question here during our panel at PAX South. Essentially it's something we have looked at, but there are currently some technical and third party issues. It's not something we are necessarily opposed to, but there are some issues that would need to be overcome.
What a strange set of imagery from the panel in the link there.
The first answer is that the reasons are "mostly technical," and that "a vast team of engineers is needed just to keep the thing running." This... doesn't sound like a glowing endorsement of the engine.
Two metaphors are then invoked: one, of a massive explosion as the whole thing blows up in your face; two, drawing on the appeal of a popular recent movie to give the technical reason for lack of moddability as "booga la booga la booga la uh ok sure just back away."
o.O
The rest of it so far is "because code, you see, and code connects to things."
None of this is unique to any one game or program. I'm pretty sure every moddable game out there is also made of code - some of it quite complex, and tied into OTHER bits of code. In fact, there are even some (a few, at least, not sure how many) mods available for MMOs, which in addition to massive, sprawling code structures for its client, has continual connection, integration, and security considerations for the back-end server architecture it's interfacing with.
One of the panel also mentions that he's "probably not supposed to look" at Dragon Age's Nexus files. Why would that be?
Overall a very puzzling reply.