Why UDHR is not legally binding?

Although the Declaration did not proclaim itself to be legally binding, it acquired such a character in two ways.

• It came to be regarded as an interpretation and elaboration of the UN Charter provisions on human rights. Its influence on the drafting and practice of state constitutions, which came into existence after 1948, has been extremely impressive and far-reaching. 
• The founding fathers of the Indian Constitution too were influenced by UDHR. Indian courts still use the Declaration to interpret and clarify the Fundamental Rights provisions of the Constitution.

Further Reading:

• Universal Declaration of Human Rights
• Human Rights Day
• Indian Judiciary