Well, the Buddha insisted on keeping things simple. He said he will only divulge the things essential for enlightenment.
The Dhamma in many ways runs counter to normal human intuition. For example, it says give up creature comforts in order to be free from suffering.
Many struggle to understand the reasoning behind such thinking even now. It does not help confusing more with complexity those who are already confused.
The Pali Tripitaka represents the bare essentials to be on the path. Although over his lifetime many of the wiser monks would have learnt and perhaps inferred the mechanics just by listening to Buddha. Perhaps a select few were taught the higher principles by the Buddha himself.
The first council did not even acknowledge Abhidhamma following Buddha’s policy on keeping it simple. Although by the 3rd council some felt it would be worthy of preservation for future generations.
An example is the 6th sense base – “Thinking”. The Abhidhamma explains the mechanics of "Thinking". It reveals thinking emerges from the heart.
Prevailing mainstream science does not acknowledge thinking as a sensory activity. It considers thinking to be a cognitive function isolated to the brain.
This has been challenged, however. It appears the heart is not only a sensory organ it has a cognitive function.
Recent work in the relatively new field of neurocardiology has firmly established that the heart is a sensory organ and an information encoding and processing center, with an extensive intrinsic nervous system that’s sufficiently sophisticated to qualify as a heart brain. Its circuitry enables it to learn, remember, and make functional decisions independent of the cranial brain. To everyone’s surprise, the findings have demonstrated that the heart’s intrinsic nervous system is a complex, self-organized system; its neuroplasticity, or ability to reorganize itself by forming new neural connections over both the short and long term, has been well demonstrated.http://noeticsi.com/thinking-from-the-h ... n-science/
So the Pali Tripitaka is akin to a driving manual. The Abhidhamma is the manual on combustion engine and its principles. All you need for the journey is the driving manual. Although if curiosity arises one may take a peek at the mechanics with the Abhidhamma too.