Crain's has a good description of why the Court denied certiorari on an assault-weapons case but chose to hear an affirmative-action case this term:
The assault-weapons case from Highland Park, Illinois, is a perfect example. The case came to the Supreme Court through the U.S. Court of Appeals for the Seventh Circuit. It raises a substantial and important issue of federal law. That would be enough for justices who think the Illinois law is unconstitutional to grant cert if they really wanted to, as Justice Clarence Thomas made clear in his dissent from the decision to deny.
At the same time, there was a perfectly ordinary bureaucratic reason for the Supreme Court to deny cert: There's no conflict among the different courts of appeal on the legal issues. Several states have similar gun laws, but so far no federal court has struck them down. In October, the Second Circuit upheld bans in New York and Connecticut. And as I noted at the time, the Supreme Court could potentially avoid the issue so long as the appeals courts don't split.
All this brings us to timing: Are the justices affected by something like the San Bernardino attack?
At the margin, there's no question that they can be affected.
Some of the column is speculation, but the author has a good read on Roberts and Kennedy.