They held the Presidency, the House and a Super Majority in the Senate. Now the blame falls on Conservatives?
Yes. But they couldn't count on all their people in Congress; some are from places where people are obsessed about their guns. Even Bernie Sanders hasn't been as hardline about gun control as he'd like people to think.
The same is true for the Republicans. There're some who'd like to do something constructive, but many more whose electorate is opposed.