Well, the devşirme is also initiated in Anatolia and the Caucasus from time to time. However, the reason of why the Balkans is the main source of recruits is because theoretically, by Ottoman jurists supporting this practice is because the Balkan area has been conquered by force or the Balkans inhabitants had only converted to Christianity after Prophet Muhammad's mission, so they didn't counted as dhimmis. There were legal scholars and jurist that's against this idea of course, as it's essentially a violation of rights of the non-Muslim subjects and created a much disputed legal problem, but usually this kind of plea got fallen into deaf ears