Because it contains all of the essential elements of a great speech:
1) It has a clear central theme — a central idea: That it falls to the living to honor those who have died for freedom and to keep the promise of freedom alive now and nourish it for future generations.
2) It also has a clear purpose: To remind us of the sacrifice of others in the name of freedom and commit us to its perpetuation and the preservation of our nation’s ideals.
3) The speech is simply organized with a beginning that makes a vital link with its audience (Four score and seven years ago, our fathers . . . ) a substantial mid section containing its core message and a strong conclusion (. . . that government of the people, by the people, for the people, shall not perish from the earth).
4) Finally, the speech is brief and to the point. It’s remarkably efficient.