Why did Diego Rivera painted sunset?

Diego Rivera did not have a specific painting titled "Sunset." It's possible you are thinking of one of his murals or paintings that feature a sunset, but there isn't a well-known work with that specific title by him.

Rivera was known for his monumental murals that often depicted scenes from Mexican history and culture. Sunsets could have been featured in these works as a symbolic element, representing the end of a day, the cycle of life, or the beauty of the natural world.

If you can provide more context or details about the "sunset" painting you're interested in, I might be able to help you find more information about it.