Well I can give you one reason: every new form of customisation is one more layer of balancing with which the development team have to deal. Ours have already proven themselves unable to deal with the five we had before: class(skills/combat style), weapon sub-type, stat assignment, element points, and optimal-based gear bonuses; instead bringing it to what they apparently feel comfortable with: class and restricted gear usage (with the promise of giving back a partial form of stat assignment in the future) plus materia.
So our developers throw their hands in the air and cut our numerical customisation options for balancing difficulty's sake .. do you really think it likely they'll add yet another?
Well, aside from this whole 'paint it red cuz red makes it hit harda' thing of course..