The answer? Rural and non-urban districts tend to elect Republicans, and urban centers elect Democrats:
"The two parties generally fight it out over the suburbs. In essence, the base of the two parties is becoming increasingly split in spatial terms: The Democrats' most vocal constituents live in cities, whereas the Republicans' power brokers would never agree to what some frame as a nightmare of tenements and light rail."
Freemark has some excellent graphs over at The Transport Politic showing just how split the nation is on urban/rural lines and how that correlates to political party preferences.
Freemark writes, "Republicans in the House of Representatives know that very few of their constituents would benefit directly from increased spending on transit, for instance, so they propose gutting the nation's commitment to new public transportation lines when they enter office."