It's related to funding, but recognition is really important. Actually forking (as opposed to the GitHub fork button) is generally seen to be hostile and and will split the community (see e.g. ffmpeg vs. libav). This means that getting a fork to succeed is much harder than it would have been for the original project. For example if the Mozilla foundation went after government grants and donations for funding Firefox instead of trying to monetize the browser they'd probably have much more success than any unproven fork could ever dream for.