Why is it necessary and proper clause in the constitution sometimes called the elastic clause?

The necessary and proper clause is sometimes called "the elastic clause" because it stretches the powers that Congress has.

The "elastic" character of the "nessary and prope" clause is that it grants Congress  implied powers beyond the specifically stated ones in the Constitution..  After enumerating a number of the powers of Congress, including borrowing money, coining money, regulating commerce, etc, Section 8 of Article I of the Constitution closes by saying Congress shall have power "to make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof."  That's a quite broad and "elastic" statement, which goes beyond specifically delegated powers and gives Congress additional implied powers.  

An example of the implementation of such implied powers in the Constitution occurred when Alexander Hamilton, as our nation's first Secretary of the Treasury, argued in favor of establishing a national bank.  Hamilton believed that anything that is not strictly forbidden in the Constitution is allowable.  A national bank was not strictly listed as something Congress could establish, but there was nothing in the Constitution to prohibit it.  And the "necessary and proper" clause gave leeway to create it, by the implied powers given to Congress.