Okay,
for some reason, I'm not surprised that Jack didn't show. But consider this. If Jin really does die on the island, then that means Hurley and Sun have something om common that the rest of the Oceanic 6 don't: they both lost someone they loved, cuz I think that Hurley really loved Charlie (as a friend of course) and perhaps they are both in mourning, and visiting that grave gives them a chance to let go of their memories of Jin and Charlie.