The main reason complex societies collapse is that complexity – like other human endeavors – eventually suffers from diminishing returns. Complex societies are problem-solving machines. They tackle one problem after another by marshaling a society’s resources toward solutions.
In the beginning, the problems a society tackles are those that are easiest to solve and bring the greatest rewards. So they are worth the effort and expense of maintaining a complex social structure. Over time, however, societies tend to move on to problems that are increasingly difficult and expensive to solve, so the rewards for solving them decline. Consequently, the cost of maintaining a huge bureaucracy, specialized professions, or upper classes grows increasingly burdensome.
The essential currency with which a society finances its complexity is energy. Energy includes food supplies, human and animal labor, fuels (such as wood, fossil fuels, or uranium), and others. As long as energy is plentiful, a society can invest in more complexity. But once energy supplies reach their limit or start to decline, complexity becomes less and less affordable.
Eventually, a society may reach the point where trying to solve a new problem is not worth the cost. From then on, it becomes vulnerable. The next crisis or serious problem that comes along may be the straw that breaks the camel’s back, and the civilization collapses.