Because in the middle of Roberts' confirmation hearings to replace Sandra Day O'Conner Chief Justice Rhenquist died and W Bush couldn't stand the idea of elevating Thomas or Scalia and he realized that Roberts was beloved of the dems (red flag!) so he nominated him to become Chief Justice before he ever served a day on the court. Of course he breezed right on through without a bit of controversy (red flag!) and has been doing the lefts work pro-bono on the down low ever since.
There was another before Roberts who was not confirmed.