Answer :
The ‘walls of the atria’ are thicker than the ventricles because the atrium or the atria carries blood from the heart that is pumped through, thereby they have a high pressure.
Whereas the ventricles carry deoxygenated blood from other ‘parts of the body’ and hence they do have so much of pressure built inside them.
The thick walls are required to bear the transportation of high pressured oxygenated blood that is being pumped by the ‘heart’ to the different parts of the body.