I kind of view it as a consequence of the market falling out from under their feet - it's cheap enough to host your own files now, and with package managers (even on Windows!) the power users that used to be the target audience of Source Forge have been vanishing.
IMHO it's more a function of their inability to compete with Github and Bitbucket. They were slow to react to the rise of distributed VCS, failed to exploit their social network features, and probably had already accumulated too much technical debt to effectively change course by the time overall trends became clear. Once developers shifted, power users had to follow suit.
