It comes down to developers and where they want to release their applications. Most developers select the all regions box but some developers have their own reasons for wanting to only display their application in one country, usually down to the content their application offers. For instance, a restaurant finder app may only have restaurants in the US, so why would this application be available in the German marketplace?
However, if you know of an app (especially a game!) which you feel should be available in your region, contact the develop and ask if its possible for them to add support for your region!
Hope that helps