I think lots of people who join end up not keeping up with contributing, and thus they never rise in level. For the rest, it is partly a random thing and partly a popularity contest, perhaps.

I see one of the commenters says he rose 11 levels within just three posts; I must assume these were hugely popular posts. Me, I almost never get any upvotes at all, and I must assume the topics I write or comment on are just not that popular, or perhaps my writing is just mediocre. 

It doesn't matter; life is not just about getting upvotes or being popular.