From what I've heard, this is because many Asian countries have a culture of information flashbang. The Yahoo homepage is extremely cluttered and busy, which fits in with what they expect.
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).
Why are you comparing homepages when the discussion is about email?
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).
in my opinion, gmail became big when it was a kind of status symbol to have a gmail address. I doubt that ever reached asia. but at one point, there were people with gmail addresses, and people with yahoo and hotmail addresses. and if you had a gmail address and you met another person with a gmail address, you knew that person was "in the know." At that point in time, switching email addresses was seamless -- you'd just switch email addresses. nowadays, you're using email for important things. even just a little bit of inertia would probably keep many people from switching.
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).