So it an undemocratic institution, which some commentary about the gun control vote noted. Senators from smaller states had more sway over the amendments tha those from larger states.
Here's one take:
The Manchin-Toomey bill received 54 aye votes and 46 nay votes. That is to say, a solid majority of senators voted for it. In most legislative bodies around the world, that would have been enough. But it wasn’t a sufficient supermajority for the U.S. Senate.
Of the senators from the 25 largest states, the Manchin-Toomey legislation received 33 aye votes and 17 nay votes — an almost 2:1 margin, putting it well beyond the 3/5ths threshold required to break a filibuster. But of the senators from the 25 smallest states, it received only 21 aye votes and 29 nay votes.
Another points out that the Senators who voted for the amendments represented almost 2/3rds of the population:
If you assume, for sake of argument, each senator represents half of his or her state’s population, then senators voting for the bill represented about 194 million people, while the senators voting against the bill represented about 118 million people. That’s getting close to a two-thirds majority in favor of the measure.
In a legislative body that didn’t give sparsely populated rural states the same representation as densely populated urban ones—and in which a minority of representatives lacked the power to block debate indefinitely—those kinds of numbers would be more than enough to pass something like the background check proposal.
A third provides graphical evidence of the ratio between the largest and smallest states over time:
And the smallest share of the electorate that can comprise a majority in the Senate: 
