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It's a combination of factors, primarily from the most common diets promoted in the USA.

Top culprits tend to be highly processed sugars and other simple carbohydrates such as starches, which are in practically everything that doesn't look like it immediately came off a plant or animal.

While most people identify sodas as the culprits, fruit juices are also high on the list as both contain high levels of acids in addition to sugars. Large quantities of either, or both, produce similar results. Bottled water also provides some negative effects since it is largely without fluoride added which is a necessary component in the development of strong tooth enamel.

Potato chips, and to a lesser extent fries, are also significant issues for teeth as they tend to form a semi-bonded coating on molars and maintain contact for extended periods of time.

A secondary cause is the oral hygene education, discipline and care, which for those without dental insurance often becomes a problem. Without appropriate basic maintenance, in the form of regular brushing and flossing, and checkups and dentist-applied cleanings maintaining healthy teeth becomes impossible.