Why did people migrate during the industrial revolution

 In 1900 it took eight (8) agricultural workers to produce enough food for eleven (11) people. By 1980 the ratio was one (1) to one-thousand (1,000). The onset of the oil age produced great advantages to a mechanized farmer. It also provided cheaper fertilizers, pesticides, and herbicides that increased productivity. This, combined with the 'dust bowl' era of Western drought, prompted a change from an agrarian to a manufacturing economy. The bottom line is that workers were needed in the city more than they were needed on the farm so, essentially, they followed the jobs. 
I suspect that one day, when oil is no longer a dependable energy source, this trend will reverse itself somewhat.