Because it's not 13 people in a room doing everything together.  

The first author listed is the primary one who did most of the work, and that was Wakefield.  The other names probably helped with designing experiments, writing the paper, or analyzing data given to them by Wakefield.  

In a non-clinical paper the last one would have been the one running the lab and the person ultimately responsibility for everything, but in this case there isn't one and Wakefield is the corresponding author, so he also the one in charge.