gym leaders are mostly ones either from family line or home town.. to keep the circle close, as you can see in-games they all know their gym leader very well.. except those who were chosen by the league. if they are chosen by the league it would mean they are very well known trainers within the league. like ex champs or elites.. maybe trainers who faught and lost but still showed some real skill against an elite or champ.
's how i see it. i could be VERY wrong but eh.