World's reserve currency (this alone should make the US #1 in "stability"- the USD fails, EVERYONE fails)

Largest military, which is primarily dedicated to maintaining global stability.

One of the few countries listed as Stable (or higher) with consistently positive GDP growth.

Overreported/exaggerated social tensions

One of the lowest unemployment rates in the world

Home to the strongest, most powerful institutions/organizations in the world (major corporations, universities, etc)

Do you want more reasons?

Edit: I'll keep going.

No tangible or possible threat of foreign invasion

Positive age demographics, relative to most of the countries above it in the list

Strong system of checks and balances within the government, so that no one individual has the power to disrupt the country (the power of the US president domestically is overblown)

The most successful heterogeneous society ever. I see Europe's having a bit of a migrant problem? Homogeneity won't last forever, wonder if the transition will disrupt anything.

World's largest economy, one of the most diverse