There are several reasons why the population of the monarch butterfly is facing a decline; the most most impactful one i believe to be the use of genetically modified, herbicide-resistant crops that destroy milkweed. monarch butterflies only eat milkweed, so the absence of this plant causes their death.another cause might be the urbanization of the zones where monarch butterflies are supposed to live and grow, making it difficult for them to live in such a different environment.