Nate Silver studies the evolution of the debate. He makes several insightful points:
It may ... be that gun control has became less a priority for the Democratic Party’s key stakeholders. On one hand, major cities where Democratic voters and donors have long been concentrated became much safer during the decade of the 2000s, and so gun violence would have seemed a less immediate threat to an Upper East Side liberal in 2008 than it would have in 1988.
On the other hand, gun control fits somewhat awkwardly into the constellation of political issues. On issues like gay rights and abortion, Democrats have advocated for a more expansive interpretation of the protections offered by the Constitution, something which stricter controls on gun ownership are arguably conflict.