It takes 4 (out of 9) Justices to grant a writ of certiorari to hear an appeal; I honestly don't know if there's a higher threshold for an appeal directly from a district court.
In theory, there is not a higher threshold. The reality, however, is that grant of certiorari to the SCOTUS directly from the district court is so rare, that the SCOTUS has apparently set a de facto threshold that is extremely hard to meet. Although I have not researched the handful of cases that went directly from the District Court to the SCOTUS, I'm going to guess that they involve imminent life and death situations involving issues of extreme importance that would probably become moot if the petitioner was first required to obtain a judgment from the Circuit Court.
Given the traditions of the Supreme Court and the importance of hearing all of the Obamacare appeals at the same time in a consolidated proceeding, I suspect that the SCOTUS's decision to deny certiorari at this time was near unanimous, if not unanimous.