The fundamental requirement for the genetic material is variability : it must be able to exist in almost infinite variety of forms.
The genetic material is able to specify a large variety of proteins. The nature of the genetic material was unknown for a long time . The cell is composed of lipids, carbohydrates, and nucleic acids, which do not have a lot of variety, and proteins which do have a lot of variety. It was thought for a long time that only proteins had enough diversity to give the orders to a cell to make other proteins. Then it was suggested that the information to build protein could be carried in a coded form. Such a coding mechanism could impart a lot of information with very simple building blocks. Consider for example that a computer codes all information in sequences of O's and 1's. With such simple coding, it is possible to display this text on the computer screen as well as diagrams and photographs.
Both proteins and DNA satisfy the fundamental requirement of the genetic material, and both are found in the chromosomes : which one are genes made of?
Where does the genetic information reside? What gives a cell orders to make proteins? Several teams of researchers worked hard to help us understand what carries the information.
These were some of the classic experiments that led to our modern understanding of biology.