There are several interrelated causes including increased population, agriculture cattle raising, and over consumption in the more developed parts of North America. People living in the area live by exploiting the forests. They sell the wood and use the cleared land for raising crops. It is very difficult to protect the forests in the face of these forces but also important. Deforestation threatens the wintering sites for monarchs and the corridors through which they pass as they migrate between Mexico and Canada.
Monarch butterflies are just one species.
This is the greatest source of genetic diversity on earth.