I have to disagree. I'm currently in Asia and /everyone/ appears to be using a yahoo email. I even created some gmail addresses for them, but it just doesn't appeal to them.
The Google homepage, in contrast, is just an empty abyss they can't get anything from. Here in the States we seem to value minimalism, which might be one of the reasons people go to Google (aside from it being a great search engine).
This is completely wrong. Most of it is just history. Gmail doesnt value-add anything to people who are content with folders for their email. In fact the labels, conversations etc confuses them. Yahoo mail in other words has less clutter, not more. (And I'm not talking of the look of the page here, but the actual email functionality).
That being said, the factors that lead to which service a person chooses for email are probably much more complex than just this.
What part of America do you live in?
Of course this list is biased - demographics matters a lot in this case. But that's also what tends to blind tech people about Gmail - Gmail for many years was massively overrepresented in the tech world. Yahoo still tends to be in the top 3 most places, and there are plenty of countries where they place well ahead of Gmail.