It's the distance.
The US parties may collude on a variety of things (like counterterrorism, or if you prefer, "counterterrorism") but they have significantly differing views on the relationship of the role of government to the citizenry and the economy. For instance, on the national level, the US Democratic party has been pushing for things like the recent health-care reform laws (for good or ill), additional environmental regulation, increases in the minimum wage, and other increases in taxes and spending which see the government taking a larger role in the economy, including transfer payments (welfare, etc). They also resent military spending as a rule. The Republican party pushes for less government involvement in the economy, lower/flatter tax regimes, market solutions to issues like healthcare and wages, and a regulatory regime which is not simply less stringent, but also more streamlined where it is in fact present (and they do not resent military spending, at least not as a rule).
Things are different outside the economic arena, true, but 2008-2016's top issues were, in order: the economy, the economy, and the economy. So.