Because the USA is the most powerful country in the world and what it does affects the rest of the world.

Trumpís ghostwriter for The Art of the Deal thinks that a Trump presidency would pose a very real risk of nuclear war and the end of human life. Thatís a fear for the planet. But even short of that, Trumpís policies on trade, the environment, NATO, Putin and so on would be a disaster for many countries.

Source: QUORA