Why hasn't there been a big national message yet? Because the election is on very few radars of non-political people yet.
Here in Minnesota, the State Fair (which ends on Labor Day) is when most people start paying attention to election season. Before then, most campaigning is at county fairs (just intro stuff), being visible in the local parade, and going door-to-door and talking to voters.
In 1994, when did the Republicans announce the Contract on, er I mean with America? In mid to late-September. When you announce the main message to the campaign, don't do it too early that people get political fatigue, and don't do it too late that the message can't get hammered home.