Histories are generally written by academics. They, quite naturally, tend to focus on the main events: the wars and the struggles between leaders and their opponents (both external and internal). Whilst these are interesting stories to read, academics, by their very nature, often overlook the underlying causes for an empire's decline.
Today, as in any era, most people are primarily interested in the "news"—the daily information regarding the world's political leaders and their struggles with one another to obtain, retain, and expand their power. When the history is written about the era we are passing through, it will reflect, in large measure, a rehash of the news. As the media of the day tend to overlook the fact that present events are merely symptoms of an overall decline, so historians tend to focus on major events, rather than the "slow operations" that have been the underlying causes.
The Persian Empire
When, as a boy, I was "educated" about the decline and fall of the Persian Empire, I learned of the final takeover by Alexander the Great but was never told that, in its decline, Persian taxes became heavier and more oppressive, leading to economic depression and revolts, which, in turn led to even heavier taxes and increased repression. Increasingly, kings hoarded gold and silver, keeping it out of circulation from the community. This hamstrung the market, as monetary circulation was insufficient to conduct business. By the time Alexander came along, Persia, weakened by warfare and internal economic strife, was a shell of an empire and was relatively easy to defeat.