Why is this unfinished: " it eluded us then, but thats no matter tomorrow we will run faster, stretch out our arms farther. and one fine...

The ironic fact is that Nick's unfinished sentence is, in a sense finished; it finishes the theme of F. Scott Fitzgerald's novel, The Great Gatsby.  For, it underlines the theme of the American Dream as illusionary. 

Nick's ruminations about Gatsby's legacy and his valiant, though futile attempts, to regain his lost love, are as romantic an idea as those of the Arthurian legends. Thus, they speak to the necessity in man for myth--something to believe in beyond himself, something that makes his life worth living, the struggle meaningful.  In the end, therefore, Fitzgerald expresses clearly the illusion of the American Dream, but he does not dispute the necessity of the Americans to dream.