Why did scientists many years ago believe that protein was the storage molecule for genetic information, instead of DNA?

Many years ago scientists believed that protein, DNA is not the genetic material because of many reasons like DNA is made up of only four type of nucleotides while proteins are made up of 20 different types of amino acids. So the building blocks of protein are way high than DNA.

Most of the cellular function is performed by proteins, not DNA. The three-dimensional shape of protein is much higher than nucleic acid. So as protein heterogeneity was high therefore it was considered as a molecule of genetic information instead of DNA.