Mostly economics...not only raw material but manufacturing costs. CF hits the sweet spot for weight, flex, strength and cost. Ti is available as noted by the examples above but typically at higher cost. As to stiffness...have to be careful about considering any material properties in isolation. It comes down to stiffness relative to section or mass. Steel has twice the modulus of elasticity that Ti has or is twice as stiff. But thinning steel out to make it comparable to the weight of Ti or at least close to it...stiffness relative to section really goes up for Ti. Same for Al which is regarded as typically stiffer then Steel but from a material standpoint the opposite is true. Al has 1/3 the modulus of elasticity that steel does. In summary it comes down to a balance between modulus of elasticity versus yield strength....objective being to develop as light a product as possible that will not permanently deform (yield strength). Overall stiffness or elasticity is as much a function of resultant wall section as it is modulus of elasticity.
George