Primarily, because it's sort of a dumb concept. America is already the greatest country in the world.
It'd be like saying let's make the Pacific the biggest ocean again! Dumb, because it is already the biggest ocean.
Secondly, because it's the campaign slogan for a certain dolt who has been exposed as being a hateful, lying, cheating, manipulative, and overall mean-spirited criminal scoundrel.
No surprise, many people aren't fond of that silly slogan.