The simple answer lies in the physical property of the material called Yield Strength.

Mild steel (plain carbon steel) like AISI 1010 has a yield strength of ~305 MPa depending on temper, while AISI 4130 chromoly has a yield strength of ~760 MPa.

In order for a frame or other structure made of mild steel to have the same resistance to permanently bending that chromoly has, the wall thickness of the tubing has to be thicker, and therefore heavier.

Considering the labor to build a frame, the cost difference between 1010 and 4130 is insignificant.