Why do people have faith on the constitution prepared from Constituent Assembly?

Because in theory they know well the requirements of the people and nation.

Explanation:

The constituent assembly is a group of politicians, professionals, academics, engineer and more sectors of the society which have been voted for the people.

Everybody suppose that they are the better representative of the people and their necessities.

People have faith of the Constitution because there is a set of rules that were formed by a group of people. They were burdened with the task of building a set of rules that a prospective country must abide by during many years to come. They can only vision as so far as this. The society that would mean that the time for the changing the Constitution had come.