Eating preferences and customs are cultural and learned. Healthy eating is a matter of acquired tastes (takes some effort to acquire them, often thanks to parents' pressure) and delayed gratification. Junk food, like all sort of mass products, gives immediate gratification to simple, innate preferences: fat, salty, sugary, high-calories and strongly flavoured.
People with low impulse control and less strict education from their families are naturally drawn to junk food, and the effects compound through the generations.