The question is “why does binding energy cause mass defect?”.
For a given nucleus, its total binding energy is the energy required to separate the nucleus into its individual nucleons. Energy is required to separate the nucleons because the strong nuclear force inside the nucleus attract the nucleons to each other. A crude classical physics analogy is two masses connected by an elastic spring with the spring not stretched initially; energy is required to pull the masses apart to overcome the force of the stretched spring tending to pull the masses toward one another.
