Jack can't die because the Island is not done with him. Michael couldn't die because the Island wasn't done with him. Locke couldn't die (first time) because the Island wasn't done with him. Sayid couldn't die because the Island wasn't done with him. Richard couldn't die because Jacob made it that way. The only reason Richard didn't die from the dynamite is because Jack was there with him. If Jack had left Richard and was safely away from the explosion, I think Richard would have died.
To me, Jacob and the Island are separate from each other. Jacob is there to protect it. He doesn't decide what the Island wants. The Island didn't want Sayid to die but you would think that Jacob wouldn't have minded. But since Jacob knows that to protect the Island, Sayid had to go to the Temple to be resurrected so he would then do the rest of the stuff he does. This somehow plays into the Island's favor. The Island wanted Jacob to die and for Locke to take over so that's what happened. Jacob was sad about it but understood he was ultimately there to protect the Island, so he died willingly. In the end, the Island will be protected by whichever candidate comes out on top. The Island set this whole thing up to rid itself of Locke for good.