Because he is being put with the Worst Generation. That's like asking why isn't Capone Bege beside Big Mom.
But that's it. he isn't, he is put right below the four emporors with the Shichibukai, We have the four emperors at the top, then below them the Shichibukai, and at the bottom we have the Worst generation bunched together, but X drake on the same line as Hancock (who is the first Shichibukai, not the one closest to the Worst Gen.) with Black Beard between them. His positioning is the only questionable there, everyone else is self explanatory.