Final answer:
The water crisis in Flint, Michigan, is mainly caused by old infrastructure, specifically aging lead pipes corroded by Flint River water, causing lead poisoning.
Step-by-step explanation:
The main reason why there is a water crisis in Flint, Michigan, according to Dr. Mao, is old infrastructure. This crisis emerged from a series of events including the switch from Detroit's municipal water to the corrosive Flint River water in 2014, primarily to save money, and the subsequent leaching of lead into the water supply due to the corrosion of Flint's aging lead pipes. This decision, influenced by systemic issues such as racism and financial pressures, led to widespread lead poisoning, especially in poorer, African-American neighborhoods. This contamination resulted in elevated blood levels of lead and various health issues, particularly affecting children and the elderly.