The Study Report says: “Sakyamuni Buddha spoke this sutra [the Amitabha Sutra] without being asked. This is because he was deeply compassionate and wanted to help all beings at the earliest opportunity, but as the Pure Land teachings were very profound and hard to believe, no one requested the teachings.”
This is a teaching that Sakyamuni Buddha gave without being requested to. There are very few such self-spoken sutras because usually a sutra was the Buddha’s answer to somebody’s question. Although no one knew to request this Dharma door, the Buddha saw that the conditions of some beings in the assembly had matured. In other words, these beings would believe and accept this Dharma door and thus benefit from it.
It is often said that a Buddha cannot help those who have no affinity with him. That is, he cannot help those who do not believe him. Therefore, when the Buddha saw that among his students there were some who could believe, understand, and practice this Dharma door, he taught without waiting to be asked. This is “because he was deeply compassionate and wanted to help all beings at the earliest opportunity.”
~ From Ven. Master Chin Kung's 2003 lecture series on the Amitabha Sutra