It is being run by a group of people who don't seem to have any useful experience of this kind of large endurance event, and your previous events haven't been too well attended or successful either.
Perhaps you should run a shorter 2-3 hour race at a popular combo to ensure you are able to get a good number of teams, and also to see what it takes to smoothly run an endurance race. For example, you have not even mentioned a tracker, and signing up by email normally doesn't generate extra interest.
I think maybe there is an issue with your sign up method as we signed up but I guess you didn't see that. In my opinion, open another thread and let teams sign up via that, it would generate more interest.