Using lesser grade material in the stays and fork blades is a cost concession to hit a lower price point. The weight penalty is not as much as most people think.
CrMo’s potential weight savings cannot be fully realized in the stays and forks because the frame would be too whippy, with more flex at the bottom bracket and less predictable steering. Stiffness in a round tube is a function of the material’s modulus of elasticity, its outer diameter and its thickness. In the case of CrMo versus1020 hi-tensile, the differences in the modulus of elastic are negligible and the stiffness becomes primarily a function of diameter and wall thickness.
A main triangle can maintain good stiffness, primarily due to the larger diameter tubes. In order to maintain the necessary rigidity, CrMo stays and blades are typically about 80% the thickness of hi-tensile stays. You lose about ½ the potential weight savings of CrMo, to maintain adequate stiffness. This is why designers often substitute lesser grade material in the stays and forks. It saves money without affecting the weight as much.
In some cases. the added stiffness can be desirable, especially with very heavy or powerful riders or in certain types of bicycles, such as a grand touring bicycles which carry heavy loads in addition to the rider.