Question

Why are the Directive Principles of state policy referred to as the “Conscience of the Constitution” ?

Answer 1

So, before coming to the answer let's understand what is one's conscience responsible for?And the answer is that it guides is towards right path.
Same task is performed by the DPSPs mentioned in the Constitution. DPSPs contain directions which any government need to keep in mind while formulating any policy or taking any action, in order to ensure welfare of the people. And hence, DPSPs serve the purpose of 'guiding light' or 'Conscience of the Constitution'.
Hope it helps.

Answer 2

Since these Directives contain guidelines given to the government to ensure the welfare of the people, which are meant to promote a just and social order, the Supreme Court has referred to them as the ‘Conscience of the Constitution’.