There are a lot of studies on this. Factors that play into this include the ability to micro target these areas with marketing, especially unorthodox methods. Large populations in close proximity lead to more ‘group think’- it is natural in large groups to go with the flow. Think of large schools of fish. The suburbs are moving further and further out of the main city and into surrounding cities (Think Chandler for Phoenix or Garland for Dallas). There is also the exposure to more services liberals push so it becomes second nature to accept its existence. Public transportation is a good example. You are dependent on government services to get around. Government can control your movements simply by accessibility of these services.
Rural and smaller towns are just the opposite. You are more independent, free to move, and less likely to formulate your opinions based on what your neighbors think. If you want to do something as simple as go across town, you are not dependent on government services and timetables to make this happen. The smaller towns you are in, the more generic political messages become due to the cost/return factor.
Other factors would include the prevalence of academics within the cities and that it is easier and cheaper for Democratic candidates to reach voters in cities than it would be for Republicans to reach voters in rural areas.
Technology such as the net and social networks should be able to bridge the gap for Republicans, but the GOP seems to be far behind the Democrats in utilizing those technologies.
I don’t know how much I agree with that. My experience has been the complete opposite. I lived in a very rural East Texas town for a decade...worst ten years of my life. Conformity and uniformity ruled the day in East Texas. Everyone dressed alike, thought alike, spoke alike, etc. Now in DFW, there is much more diversity of every kind.
If the rural areas, tired of being dictated to by urban centers, just said - ‘grow your own food’, or ‘make pianos and furniture from your own trees’, or make your subway trains and tall buildings with steel made from ore mined from your city’, etc., etc. the urban centers would have a problem.