Because it is much easier to put someone on the Cautionary list than it is the Good Trader list. Before someone goes on the Good Trader list, their refs are checked. Which takes time. For someone to get on the Cautionary Trader list, a pattern must be established that they have not been good at trading.
And no, a Cautionary Trader is far far more important than a Good trader. A Cautionary Trader can be someone on their way to becoming a Bad Trader and everyone should know that. A person can be a Good Trader simply by posting thier refs. Sure it may take someone 2 minutes to find their list, but it beats making a deal with someone who could rip them off!