It is because of the thick vegetation mainly. However, there are different categories of tropical rain forests and some of them have seasonal climate variations. For example, a monsoon tropical rain forest will experience wet and dry spells during the rainy season and summer respectively. The days are the same also because the rain forest climate is a self-sustained system. The evaporated water is less in the atmosphere above a rain forest because of the thick vegetation but water transpires in great quantity from the tree leaves to the atmosphere. This water is sufficient to create rain clouds and the same water rains back to the forest. This cycle is eternally repeated. Hence the stability and uniformity of the climate.