Well, either way you're in a vanishingly-unlikely future. I think it's that we don't appreciate the unlikelihood of our own existence to some extent - being alive, we expect that event to have been somewhat expected. In the branch where you die with every tails throw, going by quantum immortality thinking, you expect to observe yourself throwing only heads, and you fail to internalize how much this diminishes your measure - ie. you don't account for your failing branches. In the branch where you may throw tails without dying, the reasoning goes, you would have expected - almost certainly, in fact - to see a fairly even distribution of heads and tails, so the event of seeing no tails feels unlikelier there, despite having the same likelihood.