Because it's the largest, most dominant one by pretty much any metric you can think of: area, population, GDP, military influence, cultural influence, whatever.

Better look at the sovereign, the rest of the western democratic world stands on the shoulder of the United States military

Because its easy work, especially if you want to follow already set narrative.

Why show how things can work in capitalism if population is smart, hard working, moral, and government is small and effective? With parts having tons of independence? Thats literally the opposite of their system.

You can't bring the proverbial evils of capitalism when talking about happiest societies in the world, which both are based on capitalism and personal freedom.