A democratic election requires that the elected be your employee, where you work with him on a regular basis to direct him in his job. That works (ish) in government where people doing the hiring have heavily invested life interests in it succeeding.

Does a subforum offer the same? Once the mod is elected, are you going to sit down with him each day to make sure he is doing the job to your wishes and expectations? I say (ish) in government because it often doesn't even work there, even where people have heavily invested life interests, with a lot (maybe even the vast majority!) of people never getting involved in democracy. A subforum? Who cares?

If there were to be elections, it is unlikely they could be anything other than authoritarianly, with the chosen one becoming the ultimate power.