Because the cliff keeps them from quitting during the first year. If there was no cliff and vesting started immediately, the exercise window wouldn’t help much,

Because the cliff keeps them from quitting during the first year. If there was no cliff and vesting started immediately, the exercise window wouldn’t help much, since there would hardly be any options to exercise. But after the cliff, since 25% have vested, suddenly the exercise window becomes a reason not to quit.