The spall come from when people were taking the same steel that targets are mad from and cutting that to plate size. They are two different materials and the regular steel plates will not dissipate the shock cause by the bullet. That is another reason that they are not good. Spall is nothing when you are bleeding internally because of the transfer of kinetic energy.
From the explanation I was given a long time ago (for what it’s worth) the steel tends to spall on the back side more from the improper chemical makeup of the alloy rather than one being “stronger” than the other. As all steels are alloys of some kind or other, the chemical makeup from the induced alloy metals will be uneven at the molecular level causing weak spots overall leading to the fracture at the point of impact even in the most properly treated and refined materials.
Properly manufactured and refined titanium and other non-ferrous metals generally are more pure and will fracture along the entire length of whatever size/shape they happen to be due to the lack of high/low strength areas that make up steel alloys. In turn, the steel “fractures” at single points of impact where the chemical makeup that gives the alloy its strength are not as properly mixed.
Titanium will spall as well, just depends on the alloy and manufacturing process. Again, how it was explained to me a long time ago.