why does the ventricle relax as a closed chamber in the early phase of its diastole?

The ventricles relax as closed chamber in the early phase of its diastole because the pressure in the ventricles drops from the peak that it reaches in systole. The pressure in the ventricles is below the pressure in the left atrium. So, the mitral valve opens, causing accumulated blood from the atrium to flow into the ventricle.