I think osteoporosis in the West is the result of many factors. Calcium consumption for the majority of women is lower than what it should be, and other vitamin and mineral deficiencies may contribute, as well.
Perhaps, it's because of lack of exercise.
It may even be as simple as teenage girls trying to be thin; bone mass peaks in adolescence, and if girls are restricting food intake, they can compromise bone health for the rest of their lives. If they lose their periods, that exacerbates the problem even further.
Smoking, like alchohol, also reduces bone density.
There's also genetic factors, too. African Americans seem to use calcium more efficienty than Caucasians. In focusing on Western countries of both Europe and North America, Caucasians are the majority and include some enthicities that have even lower average bone densities, such as Mexican-Americans and Hispanics of Central and South America. Asians have lower bone density too, one theory being that they are generally smaller in overall size.
Therefore, it's hard to pinpoint exactly why a group of people may develop osteoporisis more than others. The reason could be diet, heredity, lifestyle, or all of the above in varying combinations.