A newbie can't set signatures (or only very short ones), so there's no point in letting them join signature campaigns. It also encourages account creation spam, so the minimum is usually Junior Member for these two reasons.
Other reason why a newbie cannot join a signature campaign, its because mostly newbie is still familiarazing and learning so the leader of such campaign not yet trusting you. experience is really acquire or to be rank up into jr. Member . most of campaign require a jr.member .