In the sixties, during the days of the counter-culture, trust in government was at a historic high of 73%. It began to fall with the aftermath of Vietnam and Watergate, rose slightly again under Reagan and Clinton, and now has bottomed out at 26% under Pres. Obama.
A psychology interpretation: blame the media. Maybe government has been basically the same over this time period, but journalists have gotten more and more tools to muck-rake on our leaders. So representativeness bias leads people to think government is less trustworthy than it really is, because that's what they see on the tube.
A political science interpretation: this is old news. People commonly say they distrust government in general, but when asked about their Congressperson specifically, they tend to have favorable impressions. This is why incumbents can keep winning even when Congress as a whole has a ~10% approval rating. It's always "the other guys" we don't trust, but of course our state would only send an angel into office.
A demonstrated preference interpretation: who cares what people say to a pollster about their opinions of government? Their voting decisions (and continued willingness to live in this country) show that they are generally pleased with what government is doing. If the median voter truly distrusted government this much, they wouldn't keep voting for it.
The Washington Post takes a lamenting tone, talking about the "death of trust in government." Frankly, I'm more optimistic. If distrust makes voters more watchful of our representatives, maybe they'll start doing more things that we want and less of the things we don't.