Probably because when Trump said it, he was explaining how America could be made better.
You seem like one of those people who thinks that America is terrible no matter what.
Also, the slogan was LET’S MAKE AMERICA GREAT AGAIN and not AMERICA SUCKS, huge difference.
just to qualify: america does suck
Nah.
But it isn’t living up to its (enormous) potential.
Is there any country currently living up to its potential?
Some countries are closer than others. America, as a continent-spanning superstate with abundant natural resources and a major high tech imdustry, has more potential than other countries, and thus has a bigger gap to try to live up to.