Democrats lack the political will and courage to take on this issue.
Gun control is alot like the Civil Rights issue....it's basically another gigantic problem the crusty Boomers will punt to future generations to deal with in the way Civil Rights was ignored for nearly a century and always deferred to some future generation to fix.
They don't lack the courage. They lack the power to do something about it. That was just as true in 2012 as it is now. So long as Republicans have a say in any part of the process, they will say no. No doubt a lot of Democratic Congresspeople recognize the futility of this so long as Republicans control a branch/chamber of the federal govt/Congress.
The quality of the legislation matters. The Democrats could scrape by the votes necessary to pass weak sauce legislation that has tons of loopholes, has weak enforcement measures, and does little to actually fix the problem but gives them cover to say they did something. But comprehensive, far reaching legislation that leaves no room for anything to slip past is a long ways away. It's always been like this in American history, we always pass weak legislation until the opportunity opens up to jam through the real deal.