With the addition of Housing in the game coming this February Master Writs are also added to the game. The chance of this dropping will depend on your dedication as a crafter. Here is a note from the PTS forums from a dev regarding what affects it as it is currently on PTS:
"Tradeskill mastery (which determines your odds of receiving a master writ from a top tier writ reward box) is meant to imply a long-term dedication to the craft. This varies from tradeskill to tradeskill.
For Blacksmithing, Clothier, and Woodworking, this means motif knowledge and overall completion of trait research. This does not include Crown-exclusive motifs or motifs for the 9 base player races. It focuses on motifs that take some effort to learn – like Xivkyn, or Minotaur, or Celestial.
For Provisioning, this is instead your collection of known purple and gold recipes as they are a strong overall representation of dedication to craft.
With Enchanting, we instead look at the total overall rune word translations you’ve completed on that character.
And with Alchemy, we look at how many reagents from which you’ve completely learned all effects.
Over time, the contributing factors for this may expand as the associated tradeskill mechanics do."
So all that time learning recipes and motifs is finally going to pay off for the Master Crafters Get collecting today!!