Thinking outside the box is all well and dandy, but the problem is you can come along with all kinds of wildish and creative ideas but this is a business, with things like finances and development time that needs to be considered. Variety is good, but it also comes with far more work and a balancing nightmare. SE has been adamant that they want everything to be balanced, to the point a lot of jobs are very similar to each other, so that no one job is left out of content. You start trying crazy things and you quickly run into situations where some things are far better than others. So you can say it's a lack of creativity, but at the same time, it's addressing the situation with a realistic viewpoint on the necessary drawbacks which come with left field ideas.
In this game, tanks = heavy armor. If they want to add more tanks while also preventing a much larger work load for their staff then tanks will continue to wear heavy fending armor. So following that logic you look at which requested FF jobs also have a history of wearing heavy armor and then your list gets pretty small considering most of them are some variant of knight. Samurai happens to be one with a history of heavy armor as well as defending/protecting others while still offering a unique flair to it, and thus it's a pretty logical choice given what few options you have.
So again, in a perfect world, every class would be wildly crazy and different, and it's fun to come up with those kinds of ideas, but if you're going to look at things seriously and take in the actual factors, it becomes a lot more cut and dry.
You're right, Auron wore robes mostly, but his "katana" was also a greatsword as well. But even then, let's take a look at Auron.
He was the guardian of summoner Braska, and caretaker of Tidus later. Auron's specialty are the break abilities that lower enemies' stats, and guard abilities to shield his comrades from enemy attacks. His early stat growth is based around high Strength, Defense and HP.
Hmmm....Defends/shields others, has high STR, VIT, and Defense...what role of the trinity has emphasis on those traits I wonder? So he may be lightly armored in appearance, but in practice he's definitely in line with being a tank. The same goes for Bravely Default's Swordmaster.