Despite efforts to improve the quality of tap water, Mexico is the world’s largest per capita consumer of bottled water, using it even for bathing babies.
People buy bottle water for one of two primary reasons. First, they may feel that the bottled water is tastes better, is heathier or in some way superior to regular water. The second reason is more of a sinister reason: they fear the quality of the tap water is so bad that it will infect them with diseases. Although American tourists still joke about "Montezuma's Revenge," Mexico has greatly improved the quality of their tap water, but it is regionally contingent. It may be safe in one neighborhood but not another, so most Mexican avoid public water sources altogether. Public confidence in public utilities and an more uniform track record of healthy water is the only thing that can change this cultural/economic habit.
Via Seth Dixon