Because they are under no obligation to continue contributing, and if the organization is serious about the future of the project and spot some talent, they'll want to try and get that person into a more permanent role. The real question is actually the opposite of the one you asked: why wouldn't they want to try and pay you for your work if it's beneficial to the product/company?
Plus you may not be working on exactly what they need from the project, but you already know the domain, the code base and the people. Even if they let you keep a few hours to do the stuff you were doing before, it's a net gain.
