The title comes from the original source novel by Thomas Keneally, first published in 1982. It was actually only titled Schindler's List in the U.S. In Europe, it was Schindler's Ark. The title was in place by the time Spielberg got to it, so that's the one he ran with.
More importantly, the list itself is the purest summation of what Schindler did to save the Jews in his factory. It becomes a potent symbol of Jewish survival in the face of a concentrated, industrialized effort to wipe them out.
So the title encompasses the two most important parts of the story: the horrors of the Holocaust itself and the way two men found to keep alive what the Nazis tried so hard to destroy. If we were telling a story about the Holocaust, we couldn't think of anything better to sum it all up.