Many have often quoted interviews as to where Yoshida has claimed Samurai would better fit the DPS role, however as I personally have not been able to find the quote, it has always been somewhat suspect to myself (not saying it is non existent, I simply have not been able to verify). What information I have found is in the video link posted below, specifically 22:15 begins a break down of how each job is not only chosen by the Entire team, but also specifically mentions that out of Two possible Tank options both DRK and SAM were in essence already designed for the role.
https://www.youtube.com/watch?v=VpxsH2rBGuw

Because of the European themes DRK was chosen this time around. Concept art, skill use, model renders, all of the development resources that went into making the Other Tank option, (Samurai) are still there, time that need not be wasted shoehorning another job in to a Tank role when one has already been made.

I have to wonder how the culture that actually has the Samurai tied so intrinsically to it's history sees the job represented. Surely I see the European Knight, the Warrior who comes from all walks of life, from every region, and the Dark Knight of actual and fantastical history, as protectors. Why the sudden disbelief that a Samurai was somehow unable to stand in the vanguard with these mythical icons simply astounds me. I could be wrong but did Samurai not have a Bushido code similar to Chivalry that stressed Morals, Honor, and Loyalty until death? The idea of seeing Samurai behind the enemy jockeying for positional set ups simply seems ludicrous.