Why is the President of India only the nominal executive ?

President of India is called the Nominal head because -

1. He is the figure head, all the decisions are on his names but the real powers are with the Prime Minister.
2. He is bound to act on the advice of the council of ministers. If the president does not wish to accept any advice he may refer it back to the council of ministers. But when the bill comes to him for a second time, he is bound to give his assent to the bill.
3. Power of the President are also not absolute. President can only declare a state of emergency only on advice of council of minister.
4. When an ordinance is issued by the president , it has to approved by the parliament in 6 weeks failing which the ordinance becomes null and void.