The most accurate reason for the significant loss of livelihoods among Mexican corn farmers as a consequence of NAFTA (North American Free Trade Agreement) is that "Mexican farmers could not compete with government-subsidized U.S. corn."
When NAFTA was implemented in 1994, it eliminated tariffs on agricultural products traded between the United States, Canada, and Mexico. This resulted in an influx of U.S. corn into the Mexican market, which was often produced at a lower cost due in part to the substantial subsidies provided to American farmers by the U.S. government. As a result, Mexican corn farmers found it increasingly difficult to compete with the cheaper, subsidized corn from the U.S.
The flooding of the Mexican market with inexpensive corn led to reduced prices for Mexican-grown corn, undermining the economic viability of many local farms. This situation forced numerous farmers to abandon their agricultural activities altogether, contributing to significant unemployment and rural poverty in regions that heavily relied on corn farming.
In summary, the primary impact of NAFTA on Mexican corn farmers was their inability to compete with subsidized U.S. corn, leading to severe consequences for their livelihoods.