Because stiffness and strength are two different characteristics of materials.
A fiberglass fishing pole rod is not very stiff at all, but it is quite strong.
Stiffness is a measure of how much (distance) a part will deflect under a given load (force). Strength is usually defined in one of two ways. One is yield strength. This is the load something can withstand without permanently deforming. Once the load is removed, if the part returns to its original shape, then it has not permanently deformed. Two is tensile strength. This is the load a part can withstand before it tears apart.
Unless you are installing subframe connectors to improve the crash safety of the car, you really don't care about their yield or tensile strength. You do care about the stiffness of the connectors, if your goal is to make the car ride and handle better. You are trying to stop chassis FLEX, not permanent deformation or tearing.
Chro-Moly steel does have much higher tensile and yield strength than mild steel, but that doesn't really matter here. The stiffness (modulus of elasticity) of Chro-Moly and mild steel are about the same. If you make two sets of geometrically identical subframe connectors, one out of each material, there will be no difference in the stiffness of the two chassis' once they are installed.
Does this make more sense now?
Yeah, I see your point now, thanks. I guess it was the higher tensile, and yield I was thinking of, not the stiffness (which in this application is the thing that matters).