There is no singular reason for the deep division that besets our nations and our churches. However, in many ways, the church is reaping what it sowed through some of the principles of the church growth movement, namely the homogenous unit principle.
This principle asserts that, since people are most comfortable being with people just like them (who share affinities, interests and demographics) we can most effectively grow churches by helping people feel the most comfortable.
This is an oversimplification in practice, of course, but this thread ran deep in evangelical circles. The fruit of this, then, has shown us to be more segregated than ever, from race to age, from political party and socio-economic class to music style.
The recovery will be long and difficult. However, through the power of the Holy Spirit and her wisdom, the church can and must work redemptively to repair these breaches. It will take intentionality and it may be painful. But we must remember both our shared humanity and divinity, best expressed in the unity found only in our diversity.