Among the other achievements of Reconstruction were the South's first state-funded public school systems, more equitable taxation legislation, laws against racial discrimination in public transport and accommodations and ambitious economic development programs (including aid to railroads and other enterprises).
It was so the delegates would be free to say what was on their minds. If anything about what they said would go to the public, there would be people who would be mad at them and thus possibly hurt them. It was to protect them.
The Black Death killed millions of millions of people which lowered the population
