What is the main reason so many great society programs became law during the johnson presidency?

The main reason for so many Great Society programs becoming law during the Johnson presidency was the Democratic majority in Congress during those years.

Explanation:

Great Society was the name for the social plan of American Democratic President Lyndon B. Johnson in 1964–65. The social plan contained a number of programs whose main objective was to end poverty and racial injustice. New major programs would address education, healthcare, urban issues, rural poverty and transport.

The Great Society closely resembled President Franklin D. Roosevelt's New Deal. For the implementation of his program, Johnson could count on the great Democratic election victory in 1964, as a result of which many Social Liberal Senators and Representatives ended up in Congress.