This has been discussed at length.
Essentially the game is balanced towards 4 man crews and the dev team has enough work to do keeping up with the current issues surrounding balance, not to mention their work on the continuing development of the game and adventure mode.
Also, practically, it can be difficult enough to cobble together a crew of 4, let alone 6. Which means at maximum, you'd need 36 people in a single lobby! Even on a Galleon, two people can generally man all of the guns worth firing. One if you've got an amazing gunner (this is a little more dependent on the guns you place down there).
As a captain, the though of coordinating more than three people why trying to fly a ship even more brick-like than the Galleon or Pyra is just not my idea of a good time!