Because human society is limited in the means of its self-perpetuation. Scarcity defines societies; there are hard technological and ecological limitations to the carrying capacity of any one civilization at any one given stage of development. This carrying capacity can expand with material progress - or retract with economic calamity - but can never be exceeded on its given level. Consequently, priority must be given to those capable of further improving the material conditions of civilization - must be given to us, the living.
Explanation enough, dear sir?