Complex sentence: Some explanations may be at first their respective population and size of music market (first and second worldwide respectively), but also for Japan by the fact it has the tradition to never release singles from album after the release of it, but only before.
Split into simpler sentences: Some explanations may be at first their respective population and size of music market (first and second worldwide respectively). An other explanation for the Japan strong presence is the fact japanese labels have the tradition to never release singles from album after the release of it, but only before.