Here is the statement from CNN:
The three debate partners, closely following Federal Election Commission guidelines, established objective criteria to determine who we invite to the debates. Because there are literally dozens and dozens of declared presidential candidates, most of whom we have never heard of, we have to have a method of determining who is invited. Our criteria simply identifies candidates that have measurable public support for their campaign. Because Mike Gravel has not demonstrated measurable public support for his campaign to date, he has not received an invitation. But we have not excluded him (or anyone) from the debate. If he meets our criteria between now and the debate, he will certainly get an invitation.