I work at SourceForge, and can probably describe a bit of the history.
In 1999 when Sourceforge was released, it was conceived as a gift back to the open source community, and was released as open source software. But with the dot com crash, and the successive transformation of VA linux from a hardware company to a software company (VA Software) the then management decided to try out a proprietary software sales strategy, and created SourceForge Enterprise Edition.
While things were proprietary due to that decision, much of the sf.net internal code became very tied up in the specifics of our infrastructure, and in some cases with code that we weren't able to open up.
All of this happened before I came on with SourceForge, and my experience is of a very different company, that has been continually increasing our commitment to give back to open source, and have released a number of smaller projects. And then a year ago we started this new Python based platform, which was intended from day one to be released under an open source license.
Trust me, we know that previous management actions have burned some bridges, but still feel like our obligation today is to do what's right for the site and the community. And I am convinced that the open source community deserves a truly open forge platform, where they can see the code, influence feature decisions, and even self host if that were to become necessary. And I know the current team is very committed to making that happen.
The decision doesn't have to be logical; it was unanimous.