This is a fundamental question that we need to ask and ultimately answer. Professor BJ Fogg, founder and director of the Persuasive Technology lab at Stanford University, has done an excellent job of describing why persons take a particular action with the Fogg Behavioral Model (FBM). It is a tool that designers can use to understand why users are taking or not taking a particular action or behavior.
According to the FBM, if your users are not taking a designed action, it's because something is missing from the behavior equation. The FBM behavior equation states Behavior = Motivation + Ability + Trigger.
When there is an adequate level of motivation coupled with an appropriate level of ability and the appropriate trigger, a behavior happens every time. This can be seen from the following graph:
At the point that these two concepts—motivation and ability trigger—converge, behavior occurs.