Market forces have to be added to the equation, if a set or theme isn't selling well in a particular country, they're probably not going to keep it on the website as long.
Simple rule - if you want a set and it's available, but it. Don't wait for prices to drop, or for it to disappear from a different country's website. You've probably got a safe year from release date, and probably two, but after that you're pushing your luck. Remember how the Vikings disappeared so quickly, despite their apparent popularity.