The Japanese Constitution was made when the US occupation army was still in control of Japan after its defeat in the Second World War. The Japanese constitution could not have had any provision that the US government did not like. Do you see any problem in this way of making the constitution? In which way was the Indian experience different from this?
The problem with making a constitution that is influenced by external factors or occupants is that it does not represent the will, goals and aspirations of the citizens of the country. Citizens have little participation in the type of government and institutions that are imposed upon them.
It also compromises on the sovereignty of the country. Thus, in many ways, it is an enforced constitution.
The Indian experience of constitution-making was different as it was based upon ideologies that were influenced by nationalist movement. The Constituent Assembly of India itself had many leaders who participated in the national movement and thus, were representative of the voice of the nation.
Sovereignty and freedom were the main goals of the Indian nationalist movement that also gave way to ideals of equality, fraternity, and secularism. This is the reason behind the democratic, secular and sovereign character of the Indian Constitution.