It’s because of disproportionate Senate representation in small and less populated states.
As Jonathan Cohn and Eric Kingsbury point out, this result is even more anti-democratic than it appears at first glance. “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.”
To put this in perspective, Wyoming Sens. Mike Enzi (R) and John Barrasso (R) both voted against the gun safety provision. Together, they represent a little more than half a million people. California Sens. Dianne Feinstein (D) and Barbara Boxer (D) both voted for gun safety. They represent over 38 million Americans. In other words, a voter in Wyoming enjoys 66 times as much representation in the Senate as a voter in California.