The moderator's daily job is mostly similar to the /review for which one could get the silver Reviewer badge after some good effort. Why isn't this one of the criteria for a moderator candidate? It's even not mentioned in the list of noteworthy badges at elections.se. The badge is IMO relatively easy to earn and users with the badge show off that they are familiar with the boring day-in, day-out job of a moderator.
I would imagine that it's because only 208 users on Stack Overflow would qualify.
That's a fairly small cross-section of the Stack Overflow User Community. I believe that we're looking for a bit more diversity and representation of the community than that. We want to draw from a larger pool.
The sorting-out process doesn't end when a moderator gets elected. I've only been a mod for a short while, and I've seen what I thought to be highly qualified individuals wash out, simply because... well, I don't really know why. Conversely, I've seen people get elected who made me go
o_O, but who turned out to be exceptional moderators.
Consequently, once a user has demonstrated willingness, good judgement and an enthusiasm for sweeping the floors, I'd be more apt to endorse them than someone who simply spent a month earning a silver badge.