Because there's nothing in particular that causes what's happening in the central part of the distribution. The centrally located outcomes are the result of com

Because there's nothing in particular that causes what's happening in the central part of the distribution. The centrally located outcomes are the result of combining a myriad of small factors this way and that way. Outliers usually have an identifiable, single cause. This is why in statistical process control these types of outcome are known as "common-cause variation" and "assignable-cause variation".