Akka is Open Source. Akka's authors feel that continuing the status quo is unsustainable.

Whether or not it's unsustainable because it's open source to some extent doesn't really matter – perhaps with enough elbow grease on their part they could drum up more community maintainers and make it both sustainable and continuously open source, but that would require putting in yet more uncompensated work on the hope that it might lower their burdens at some point down the road, and come with the issue that more volunteers mean more work trying to coordinate everyone.

Or they could just make it source available for the first 3 years and ask for money from the companies using it commercially – there's also no guarantee that this will work, but money will solve more problems in the short term, so I can understand why they made the choice they did.