People have been moving from the country to bigger towns/cities for about a thousand years.  Farm life and rural life in general doesn't offer as much opportunity as the city nor does it offer as much entertainment.  I suspect that there is a basic hive or tribe quality in human nature that makes most want to live among others.

Eastern Iowa is less susceptible to that as a lot of people like to live in a small town that's within driving distance of work in the city.  Cedar Rapids, Dubuque, Davenport/Quad cities and Iowa City are area's that offer decent jobs while the surrounding small towns offer that quality of life not found as much in those towns.  And have generally lower taxes.

Part of the key is a highway system that allows somebody from Maquoketa work in Dubuque and get there about as quickly as somebody who lives on the wrong side of town and has to fight traffic.