El Pais has a long article that goes through the entire supply chain for Fentanyl. Basically, there is a huge demand for drugs in the United States. Because they are criminalized and highly profitable, organized crime is going to find to meet that demand. Organized crime in Mexico is going to find a way to bring the chemical precursors in, manufacture the product, and move it across the border, with extremely violent consequences. Businesspeople in China (I use the term loosely as these are also gangster-like, but the actual chemicals involved are typically not even illegal) are happy to manufacture and ship these precursors to the organized criminals in Mexico. Politicians and bureaucrats at all levels get wrapped up through bribery, threats of violence and actual violence.