Question

why are not part of bicycle made from chromium itself

Answer 1

because chromium oxidises with oxygen to make dichromate which is orange in colour

Answer 2

Not all bike frames are created equal. Since the backbone of any bike is its frame, it can be enlightening to understand how it's made and what that means for you, the cyclist.The goal of any frame is to offer extraordinary strength with minimum weight. However, frame strength is determined by many factors :
1.Geometry
2.Weight
3.Welding.