Gyaltrul Rinpoche
The Buddha's manifestations are occurring in many, many realms of existence, appearing in countless modes, for the sake of guiding and liberating others. Many individuals on various planes of existence receive teachings from the Buddha according to their merit, but if that merit isn't sufficiently strong the teaching will eventually die out in that particular realm.
It's one thing to have the good karma and sufficient merit to receive a teaching, and it's altogether another thing to have the continuing merit and good karma to maintain the presence of that teaching, not only in one's mind, but in one's karmic environment as well.
For example, consider the situation in the present time when the merit for the Buddhadharma's continued existence in Tibet has become exhausted. For the most part, it has passed from that land and spread to Western countries. Here in America there is some merit for dharma to flourish right now, but that doesn't mean it will always be here. Its continued presence is dependent upon the ability of the people here to continue accumulating the necessary merit.