The first reason why we accept a constitution which is made more than fifty year ago is because
1) the constituent assembly represent the people. it is not elected by the people but it has so many people of different cast, language, region, gender and so on. so they have different idea. so the assembly represent the people of our country
2) the constituent assembly work in a systamatic manner, open and consensual manner
3) the consituent assembly assembly which drafted our constitution does not reflect the views of the members but the whole country.