Back to the point: It's simple math. If you plop some metro into a state and the first 10 miles in each direction from city center is 85% Dem and 15% GOP and you draw district lines there with no slivers protruding out, you've taken away so much margin from the rest of the state. Take a perfect bellwether 50-50 state and make 1 very tiny geographic spot house 80% of your excess margin and watch as your delegation is more often than not going to the other party.
Not many states are as extreme as IL where half the Dem votes come from one county.
So instead look south of MN to bellwether IA. Like MN they have a neutrally-drawn map that uses geographic principles. The congressional delegation has moved back and forth over the decade from 1D-4R in 2004 to 3D-2R in 2006/8/10 to 2D-2R in 2012 to 1D-3R this year. Their map is fair and competitive allowing a relatively equal number of both sides to prevail. In more urban OH, a neutral map that reflects a 50-50 state can be drawn with reasonable geographic principles, as was demonstrated in their
2011 competition.