The reason is that declining nations follow the same pattern, time and time again, over the centuries. This is not coincidence. The pattern exists because human nature never changes, regardless of the era or the locale. Political leaders make the same mistakes as their forebears, and the people of a nation react in kind.
For this reason, countries have a sort of "shelf life." They rise in prominence, due to work ethic and productivity. They then go through a period of abundance, which eventually deteriorates, due to complacency and apathy. Finally, they collapse into a period of bondage.