I think a reason these users are not staying is not succeeding with early posts. While obviously it is unrealistic to expect success with your first post I feel it is very hard to get any traction if you only got 5-6 SP in your account.
I am not sure what the answer is, maybe a larger community with more specialised topics and a better distribution of the voting power
I agree with that probably being a reason. But it is a rather silly one. People don't leave other social media sites because they aren't getting paid. Regardless I'd love to be able to look at new user churn.
Specialized topics will definitely help with retention. The longer a user can stay on the site the more likely they are to mention it to others.