Maybe because they hear from lobbyists more than they hear from us, which continues the whole crazy cycle? Via TPM:

Researchers from UC Berkeley and the University of Michigan dug up some surprising results after posing the question: How much do lawmakers really know about their voters' political views?

"Pick an American state legislator at random, and chances are that he or she will have massive misperceptions about district views on big-ticket issues, typically missing the mark by 15 percentage points," David Broockman and Christopher Skovron wrote in a study for the Scholarly Strategy Network originally published in 2013.

To investigate the question, the duo surveyed thousands of state legislators and compared their perceptions of voters to people's actual views, derived from a large body of public opinion data.

Their conclusion: "Legislators usually believe their constituents are more conservative than they actually are."

On three issues — universal healthcare, same-sex marriage, and welfare — lawmakers' assumptions about what their constituents believed were "15-20 percent more conservative, on average," than the actual base of public support for such issues.

This is why I'm forever urging you to write or call your representatives. The other side is well organized and always happy to pick up a phone and rant-- while our side is full of disgruntled people who say, "What's the use? They never listen anyway."

How are they supposed to know what you think if you don't tell them?

Democratic voters need to get better at the long game. Just because you don't win right away doesn't mean your call didn't count. I guarantee you there's a staffer keeping track, who tells the member, "Calls in favor of this are up 31% over three years. We should talk about this."

Yes, even the Republican legislators. They like their jobs, they want to keep them. When voters reach critical mass on an issue, of course they're going to pay attention.