Why is the story called "Dusk"?

What a great question. I believe the short story "Dusk," by Saki has its title for a two-fold reason. The setting is around 6:30 on a March evening in Hyde Park: dusk. But more importantly, I believe Saki is commenting on our inability to actually see others.
The protagonist, Norman Gortsby, is a cynical man who reckons himself perceptive, aware of the plight and motives of other people. He enjoys sitting in Hyde Park at dusk watching the people go and come. He presumes those who leave have places to go while those who come are defeated in some way.
He immediately thinks an old man sitting near him one of these defeated people, yet the old man gets up and leaves. When a young man sits next to him, indeed sounding defeated, Norman asks him what is wrong, and the young man tells him that he has forgotten which hotel he was staying in when he left to buy soap and that now he is stuck in a strange city with nowhere to sleep. He hopes some kind stranger will help him out so that he can sleep in a safer place.
Though the story sounds convincing, Norman, cynical as usual, is convinced that the young man is a clever panhandler trying to trick strangers into giving him money that he will never repay. When the young man cannot produce his bar of soap, Norman feels justified in his judgment and the young man leaves. But when Norman notices a small cake of soap on the ground near the bench, he feels terrible for having not seen the truth about this young man's honesty, tracks him down, and loans him some money. Feeling good about this faith-restoring encounter, Norman returns to his bench only to find the old man whom he had earlier presumed was defeated searching for his lost cake of soap. Ironically, Norman's enlightenment has only served to darken his outlook.
Dusk can represent a number of things symbolically, but in this case I think it represents not only Norman's inability to fully see the truth in either of the men, but also his dim, cynical outlook on life, which is only reinforced by the young panhandler's swindle that left Norman defeated. Saki is telling us to be careful whom we trust, but he's also telling us that trusting can give us a kind of joy that cynicism cannot. We are faced with a choice. Either we darken our hearts a little as we learn through experience not to trust other people, or we choose not to see so that we can believe in others. Either way, the outlook is dim, and we are always in the dusk.