When the hydrogen bomb was detonated underground pieces of dirt, mud, Juliet, trees, and metal would have been blasted into the air. Then all of this stuff would have mixed with the fission fragments of plutonium and uranium and pulled them to the ground under the force of gravity. Radioactive bits of dirt would sprinkle down in places miles from the island. This is called radio active fall out and would last even until 2007 and we'd know because the ocean current would have distributed it all over and THE WHOLE PACIFIC OCEAN WOULD BE POLLUTED WITH RADIOACTIVE SHIT! If whatever happened happened and the bomb always went off, all the survivors from 815 would have died of radiation poisoning a week after the crash and the show would only have had six episodes long and no one would watch them!
They can't continue the show if they changed the future and suddenly everyone dies. So the only way for them to continue the show without ignoring basic laws of science would be for them to have created a paradox and be reset way back!
(srry if i sound like an ass hole by the way, i'm just in a bad mood... :/