One of the reasons that it is actively declining is that people believe the only way to win in the search results is to pay for PPC advertising and outbid their competitors. SEO has the stigma of being too difficult to get long term results with the 500 plus algorithm changes each year.
The main reason these two countries hold the top positions is because many SEO agencies in the United States outsource link building to these regions because they can get thousands of generic, cheap backlinks built for very low costs. The problem with this is they are low-quality links and the clients they serve are only going to get their website hurt in the long run. Google manual penalties have taken out thousands of sites because of these improper practices.