Because it's going to take 2157 delegates to the Democratic National Convention in August 2008 to secure the nomination and you have 138 days, from the opening of the Iowa caucuses to the closing of the polls in the California primary, to do it. You're going to have to juggle campaigns in two to three dozen states (most of them concurrent), recruiting and training thousands of volunteers and field staff. In each targeted state, you'll need to contact tens, perhaps hundreds of thousands of Democratic and Democratic-leaning voters and caucus participants, and identify those who are leaning your way and those who need a little persuasion. Then, well, you need to persuade the persuadable. Then you need to make sure you get all these identified voters and caucus participants to their polling/caucus venues on caucus and primary days. If they can't get there, they'll need rides to the polls, or maybe absentee ballots.