Me? I blame TV, but that's another post for another time.
Instead, just look at the graph created by the fine folks at NES. Sure it jumps around a bit. Each election is unique. If you draw a flat line you might find a small, stumbling decline, but you can also see in the 1990s the control of Congress became a huge story, especially when the Republican Party took control in 1994, thus improving accuracy by the public. Plus in off-years the accuracy sucks because, usually, real people don't care.
The lesson? Maybe it's Congress matters less and less in people's lives. Maybe the media devote less time to this. Maybe it really doesn't matter. For me, the lesson is that when an issue gains the attention of journalists and the elite, there is a trickle down effect for the public as well. Coverage improves knowledge, especially when there is an ideological or partisan edge.
The not-so-silent hand of the marketplace of ideas, hard at work for us all.