Some think that it is due to increased use of refined carbohydrates. Others think it's due to increased high fructose corn syrup consumption. Some think it's simply due to the fact that one to two generations ago, many people did hard physical labor as opposed to sitting at a desk. Some think it's a combination of many factors.
There is an interesting paper on the increased dietary variety and the prevalence of increased BMI. McCrory MA, et al. Dietary variety within food groups: association with energy intake and body fatness in men and women.
Am J Clin Nutr. 1999 Mar;69(3):440-7.
The paper is free online, if anyone is interested.
Figure 4 is particularly interesting.