Because his teaching was teleological, aimed at actually putting an end to stress, the Buddha needed to explain only what was necessary toward that end: the ways in which past and present kamma shape experience.

"[The Buddha] listed four inconceivables — topics that lay beyond the range of an ordinary person even to speculate about in a healthy way. In the words of the passage, these topics “would bring madness & vexation to anyone who conjectured about them.” They are:

the Buddha-range of the Buddhas
the jhāna-range of a person in jhāna
the results of kamma
conjecture about the cosmos.

The inclusion of the first two items in the list serves notice that the Buddha was not putting himself in the position of an ordinary person speculating about these matters. There was much that he knew through direct knowledge — through mastering jhāna and becoming a Buddha — that he did not have to speculate about. Thus, although the entire list lies beyond the range of healthy speculation, it tells us that we cannot know the range of the Buddha’s own knowledge of these things.

The inclusion of the third item in the list, the results of kamma, may come as a surprise, given the care with which the Buddha explained the results of kamma in many discourses. However, its inclusion here points to the fact, discussed in MN 136 [§66], that the workings of kamma are complex — more complex, in fact, than is indicated in that discourse. Their complexities would have posed a challenge for the Buddha if he had wanted to construct an explanation of stress and its end based on first principles, for a theory of kamma would have been a logical place to start. Thus he would have been required to give a full explanation of how and why kamma is complex. But because his teaching was teleological, aimed at actually putting an end to stress, he needed to explain only what was necessary toward that end: the ways in which past and present kamma shape experience. Although past kamma can influence the conditions on one’s sensory experience, the actual stress or lack of stress experienced by the mind is the direct result of present kamma — the act of following or abandoning clinging and craving. For the purpose of putting an end to stress, all that needs to be known is how to create skillful kamma and then — once that skill is mastered — how to create the kamma that puts an end to kamma [AN 4:237]. Thus there is no need to account for all the complex interactions of kammic results. A knowledge of general principles is enough.

~ Thanissaro Bhikkhu "Skill in Questions: How the Buddha Taught"