This is probably related to the function of each. Atria have less muscle in the walls than ventricles do. Atria receive blood from either the vena cava on the right side of the heart or from the pulmonary veins, on the left side of the heart. This blood is pumped down to the ventricles, which are larger pumping chambers which must propel blood out of the heart therefore, they contain more muscle tissue than the atria do. On the right side, the right ventricle pumps blood out through the pulmonary arteries to the lungs, which is not a long distance. However, the left ventricle, which is larger and more muscular than the right ventricle, pumps blood out of the heart via the aorta, to all parts of the body.
The atria are thinner than walls of ventricle because of there function.
Since we known that atria receives blood from various parts of the body and ventricles pump blood to various parts of the body, pumping require more pressure and force so to withstand the pressure the walls of ventricle are thicker than atria walls.