In literature as well as in art, there are many natural elements such as the sun, the stars, the flowers, etc. that are used symbolically and also to beautify the imagery.
In the case of both the sunrise and sunset, we can understand that these represent a cycle, the beginning and the end of something or somebody.
Thus, in the case of "Because I Could Not Stop for Death", the image of the setting sun is not described in great detail because the context of the poem gives us the idea of a cycle that ends, that is to say death, and death itself does not require much description.