Because for any event or proposition it will either happen or not happen, two possible states. And you figure probabilities from the canonical fair coin and come up with two states & a 50/50 chance of either one coming up. So the proposition "A giant meteor will hit the Earth tomorrow" will either happen or not happen, so there's a 50/50 chance of a global extinction event tomorrow.
Because for any event or proposition it will either happen or not happen, two possible states. And you figure probabilities from the canonical fair coin and come up with two states & a 50/50 chance of either one coming up. So the proposition "A giant meteor will hit the Earth tomorrow" will either happen or not happen, so there's a 50/50 chance of a global extinction event tomorrow.