The aorta wall is thicker because it has a greater force of pressure to deal with from the left ventricle. The blood needs to travel throughout the whole body so it is pumped hard for the blood to circulate. The walls of the aorta need to be thicker to withstand the higher pressure.The pulmonary trunk only pumps blood to the lungs which is an area of lower pressure so the walls are not as thick as the aorta.
Hope this helps!