Democrats are much more interested in considering issues of class. In turn, central cities are Democratic because of the enormous class disparities that exist there. Residents of central cities (especially old-fashioned, high-density cities) see large numbers of very wealthy and very poor, often with no middle class. Such a situation creates anger among the poor and guilt among the rich. In both cases, the result is a Democratic landslide. Suburbs tend to be MUCH more homogenous. If you only see people who are economically similar to you, you're going to be much less likely to feel a great crisis in wealth distribution.
The suburb depiction I would say was more true in the past or now at least more true in the south & midwest. Suburbs in the northeast & midatlantic especially don't really fit into this mold & have trended Democratic over the past 15 years or so
I think that the Southern Evangelical base of the national Republican party has scared away Northeast suburbanites. These people don't want an authoritarian government, which many Southerners want, making personal decisions for them, such as telling their children what to believe (prayer in schools), whether they can get an abortion, whether they can choose to allow a relative on life support to die...
