If you can unlock the riddle as to why voters vote the way they do, then my hat is off to you, because it's difficult to do. We still don't know why voters voted the way they did in the 1998 midterm election, or the 2002 midterm, or the 2006 midterm.
Media and advertising. The more money pumped into both, the more votes you get. Unsurprisingly, the masses begin to pay attention about 2-3 weeks before the election and that's when the most money is channeled into these outlets, fueling their decisions.