Well, we've got members with 500+ posts, and if we were to give to to the one with the most posts, new members would have no chance at winning.
20 posts gives you the max entry level, so you can get the best chances possible, with little effort. Right now we've got about 10 members entered at full level, and the contest is very short (no one knows when everyone will be able to get one of these at a whim, so we're not going to wait too long
)