I heard about some reasons why open-sourcing software is a little difficult when MS open-sourced Windows LiveWriter as Open Live Writer. The reasoning is that a lot of the code/supporting tools that they use have non-permissive licenses and is also reused in other places which makes it an involved process to decouple it from the application. A good example is the Ribbon interface in OLW (which I think Scott Hanselman or someone got rewritten so that it could be open-sourced.).

Don't quote me on this though, because I wasn't a first party member of the conversations.