Why does the constitution of India contain different forms of oath for the president, the Ministers, the Legislators and the members of the judiciary?

Functions and responsible responsibilities of the president, the Ministers, the Legislators and the members of the judiciary are specific and different from each other.

Our constitution requires any person to take an oath or affirmation before entering into office or taking over the responsibility. Oath or affirmation binds the person to do his duties well and to be faithful to it. Different authorities need to perform different functions or responsibilities and so, the forms of oath are also different accordingly. However, all these are commonly obliged to bear to true faith and allegiance to the constitution and to uphold sovereignty and integrity of India, on oath.

The president is the head of the state and all functions of the country are performed in his name. Moreover, being a part of Parliament and having ordinance making powers he also plays he role of a legislator. Par doing power in the field of judiciary and supreme command of the Armed Forces in his hands, requires him to be faithful to the constitution.

Article-60 provides the form of oath or affirmation of the President. It requires him to preserve, protect and defend the constitution and law at his best ability. Also, he is required to devote himself to the service and well-being of the people of India.
On the other hand, a minister belongs to political parties. In political ups and downs he performs his duties. Therefore he is required, on an oath or affirmation, to discharge his duties without fear or favour, affection or ill-will. In the meadst of political activities, he is very much required to keep secrecy of official matters.

The legislators are to make laws and solve the problems of national or local importance arising day to day. They need to discharge this duty faithfully on oath. Functions of the judicial members are to uphold the constitution and law of the country. They take oath to this affect and accept duties to perform them with best ability, knowledge and judgment. They are expected by oath to discharge these duties without any fear or favour affection or ill-will.

So, for these reasons of speciality of functions and duties, our constitution contains different forms of oath for them.