When money is used as an external reward for some activity, the subjects lose interest in the activity.
Gamification: Motivation Model
Eugen Eşanu

It is more like: The money reward sets a clear framework or reference point, so to speak. The participants used this reference to just do as min. as needed to reach that reference point (because we know; humans are lazy and always looking for ways to save energy). Now on the other hand – if you don’t provide a clear reference point, things are left open. People keep going and a natural curiosity certainly helps, too. That’s why you get the results.