As a friend often tells me, before the Civil War people would say "the United States are", and since the war they say "the United States is".
I suspect that's because of the Union/Northern Propaganda. Part of any war is figuring out the narrative to use to sell the war to your people, both the people fighting and the people undergoing hardship at home. While that's always been true on some level, it's been particularly important since the use of the longbow in 1415 at the battle of Agincourt. (Because at that point wealth and a small number of people was no longer sufficient to win a war--mass infantry and therefore control of public sentiment became necessary to field an army.)
We have been swinging the other way for some time now, because of automation, and democracy will become much less useful to the preservation of the state over time. I'll be surprised if it survives the next Millennium. We've been raised to like it, and it has a lot going for it, but it's too inefficient in its current form probably anywhere in the world.
In any event, today's common claim is that the Civil War was a fight "to preserve the Union," and while I don't see a contemporary reference to that propaganda in a quick google search, if that was part of the narrative then the shift to "The United States Is" was inevitable, and probably started as part of that campaign.