Many reasons.
Bad teeth in general, lack of education about mouth hygiene (I remember those massive teeth models and teeth brushes the size of a club), "do not care lol" and the general high consumption amount of sweet things.
Obesity also can be a thing that could lead to having cavities, as well as other medical conditions. For when people have lost their hold on life that they even cannot brush their teeth regularly, let alone manage to get out of bed.