The answer is fairly obvious, as this article in the Ensign explains..
"The United States is the promised land foretold in the Book of Mormon—a place where divine guidance directed inspired men to create the conditions necessary for the Restoration of the gospel of Jesus Christ. It was the birth of the United States of America that ushered out the Great Apostasy, when the earth was darkened by the absence of prophets and revealed light. It was no coincidence that the lovely morning of the First Vision occurred just a few decades after the establishment of the United States."
Here's a video depiction of the promised land:
The original site has more graphics.
And here are more references from the Ensign:
Marion G. Romney
More from the first article I cited: