There are a couple of different factors in play here:

1. People tend to settle in areas where other people who are similar ideologically to them live. This silos people of similar ideological persuasion geographically.

   This has been extensively covered, e.g. in [Washington Post "People move to places that fit their politics"][1], citing [data from 2014 Pew research][2] (or [this][3]).

   Or, to quote [NPR's "How Republicans And Democrats Ended Up Living Apart"][4]:

   > Back in 1976 — the year of a close presidential election — just over a quarter of the population lived in "landslide counties," where the winning margin was greater than 20 percentage points, says journalist Bill Bishop, author of The Big Sort: Why the Clustering of Like-Minded America Is Tearing Us Apart.
   >
   > Last year, more than half the country lived in landslide counties. And, while Barack Obama's margin of victory was less in 2012 than it was in 2008, the number of states decided by fewer than 5 points actually went down.

   Of course, 538 covered this as well, in ["http://fivethirtyeight.com/features/urban-resurgence-is-a-double-edged-sword-for-democrats/"][5]:

   > The harsh reality for Democrats is that two-thirds of all congressional and legislative districts are in suburban and rural America. Bill Clinton carried both in 1992 and 1996, but Obama lost both in 2012. This is a big reason that Democrats have lost about 70 House seats and 910 state legislative seats, on net, since Obama took office.
   >
   > Back in 1988, the suburbs voted 4.5 percentage points more Republican than the nation as a whole. In the 1990s, migration of city dwellers to the suburbs made them much more diverse and helped Democrats narrow the gap to just 2 points by 2000. But millennials’ recent pattern of delaying outward moves and the resulting suburban slowdown have stifled Democratic advancement, and in 2012, the suburbs voted 3.1 percentage points more Republican than the nation.

2. "First past the post" electoral system. This has been discussed ad nauseam on Politics.SE.


  [1]: https://www.washingtonpost.com/news/the-fix/wp/2014/06/13/people-move-to-places-that-fit-their-politics-and-its-helping-republicans/
  [2]: http://www.people-press.org/2014/06/12/section-3-political-polarization-and-personal-life/pp-2014-06-12-polarization-3-02/
  [3]: http://www.pewresearch.org/fact-tank/2014/06/13/big-houses-art-museums-and-in-laws-how-the-most-ideologically-polarized-americans-live-different-lives/
  [4]: http://www.npr.org/sections/itsallpolitics/2013/11/26/247362143/how-republicans-and-democrats-ended-up-living-apart
  [5]: http://fivethirtyeight.com/features/urban-resurgence-is-a-double-edged-sword-for-democrats/