Moderator is a difficult task, as it needs a particular user being comeptent about the topic discussed in the board (espacially for local boards) and the forum rules, or at least forum etiquette, whatever that means. This restritct hte choices by theymos to an handful of users, provided that not all users are willing to take this role.
Of course this is true and aren't just assigned to someone who aren't that competent to handle it, though local board could be that easier but in general board its requires such person to be that knowledgeable towards that topic of discussion and to the entire boards.
I could remembered how our local board mod was assigned, it was based on the community choice and how active and dedicated he was on the local also attributes to him becoming one of them and I think the task is also much on him which we need more mods.