Because it is our inheritance and legacy.  It is our and nature's natural "right" to experience the awe of the natural night sky to live, to be inspired, and to thrive (animals, insects, and you and me too!).  For eons humanity has looked to the night sky for meaning, purpose, and direction.  Wars began and ended; kings were crowned; the masses were inspired to act (good or bad); prophecies were fulfilled; poems and music were written; lovers filled their hearts; and the disheartened looked for solace.  

When one looks up at the starry sky on a clear cloudless night, and realizes that this is the same night sky, and the same experience bestowed upon the first homo sapiens when they looked up, that moment of awe is profound and stirring.  

As Carl Sagan said, "We're all made of star stuff."  We are the universe and the universe is in us.  There is no separation.  To experience this unity and timeless, can only be had on a clear night when the stars and planets dance like jewels in the sky.  No one should be denied this.  It is our task today, and an urgent one, to preserve the natural night sky for future generations.  Honestly, I feel it's my duty.