I've heard one theory, and it's hard to deny. We Americans are stubborn independent fools. Citizens in most countries tend to trust and follow their governments far more than us Americans. They say all you have to do to help prevent the spread is wear a mask, most of us say you can't make me, it's against my rights.
I'm not saying it's all bad to be independent, but if you look at it from an outsiders viewpoint, you can't deny it's true. Also why it's not surprising the US leads the world in cases. We are the third most populated, but China and India do a far better job of controlling their population than our country could ever attempt. Assuming you believe China's case count anymore.