Originally Posted by koffee brown
Because he didn't have the best overall lowest time for all stages, just won for that particular day. The person with the lowest overall time gets the yellow jersey. The first 25 riders get points as they cross the finish line, and the one who comes across first gets the most points (sprinter, that is). At the end of the race, it's total points, not total time, that determines who wins the jersey.
Koffee
There are also intermediate sprint points along the routes (not sure just how many stages they're in... all or just some) that contribute to the sprinter's point total, so it's not just who crosses the finish first for the day. In fact, the green jersey may go to (or be kept by) a rider who doesn't finish in the top few riders. Though, admittedly, he won't keep it very long by not finishing in the top few riders each stage.