Why was nucleic acid disregarded for so long as the molecule responsible for the transmission of hereditary information?

Because protein was considered as hereditary material.

Explanation:

Until 1940, many scientists believed protein is the genetic material. There were two reasons for it - proteins are present more in number in the cells of animals. This amount of proteins varies in different types of cells, still they are abundant in the living cells. 50% of the dry weight of a cell consists of proteins.

The second reason was nucleic acids were considered very simple in structure. That's why it was unbelievable, they can carry hereditary characters. This was the very reason it took a long time to prove nucleic acid carries genetic information.