For Dark Knight I think it's the whole "Badass" image that attracts newer players, and for Dragoon it's the Wyvern. I also notice this, and when players decide they want these Jobs it's usually before they start the game, so it can't be too much to do with function more than it is image. That's my only theory.

Oh and maybe because of the Armour, which I must admit looks rather Awesome.