The intransigence of almost all republicans in the senate. If Obama wants it, he can’t have it. It doesn’t matter what their own constituents want. (fer chrissakes, background checks have 72% support in friggen Alabama.)
The absolute terror that most republicans and a handful of dem senators have for the NRA. (there is no possible other explanation for Heitkamp’s vote, her lie of an explanation notwithstanding.)
And a bonus third reason. Complete ineffectiveness of dem leadership from both Reid and Obama.