because newbies tends to spam or could lead to spam and abuse by users. they could possibly create a new account in order for them to participate many times in a campaign and also it  could lead to a lesser overall rewards . however  there were also campaigns that accepts newbies and most of them can be found in bounties altcoin section but there was once in marketplace services section although there might be rare and sometimes it is verry hard to join them because the managers are pretty strict.