Also, people are less active:
There's also a major increase in sedentary jobs:Americans have become less physically active overall between 1955 and 2005. While the rate of leisure-time physical activity has not changed significantly there has been a decrease in work-related activity, human powered transportation, activity in the home, and increasing sedentary activity. During 2000 and 2005 the number of adults who were never physically active increased from 9.4% to 10.3% while the number who were engaged in the highest level of physical activity decreased from 18.7% to 16.7%. Pertaining to leisure-time physical activity, people involved in no activity increased from 38.5% to 40.0% while those who spent most of their day sitting increased from 36.8% to 39.9%.
In 2000 the CDC estimated that more than 40% of the US population was sedentary, another 30% was active but not sufficiently and less than 30% had an adequate level of physical activity. There has been a trend toward decreased physical activity in part due to increasingly mechanized forms of work, changing modes of transportation, and increasing urbanization. Obesity rates have increased in relation to expanding suburbs. This has been attributed to increased time spent commuting, leading to less exercise and less meal preparation at home. Driving one's children to school has become increasingly popular. In the USA the proportion of children who walk or bike to school declined between 1969 (42%) and 2001 (16%) resulting in less exercise.
You don't need to study the primal lifestyle to realize that more calories consumed, plus less physical activity, plus less physical jobs will make you much fatter. Believe it or not, gluten is not to blame for everything.
US per capita caloric intake, 1970-2008 » Graphic Sociology
PLOS ONE: Trends over 5 Decades in U.S. Occupation-Related Physical Activity and Their Associations with Obesity