Linguistics Reason: Ananda Memorized Sutta For The Other, But Sariputta Memorized Abhidhamma That Buddha Taught Him Directly

Abhidhamma is sutta, that sariputta learned from buddha. Sariputta and buddha taught abhidhamma to whole of 500 arahanta, then 500 arahanta bless it in 1st sangayana. So abhidhamma have difference linguistics from sutta, of ananda. Ananda memorized whole sutta, that buddha have taught to another. But sariputta memorized the reasons (relation) of everything, that buddha taught him directly. Age of buddhist cannon linguistics having the same time being. Buddha taught someone, then he repeat sutta to ananda. Buddha repeat sutta with description, that ananda ask him, commentary. Each sutta description have many causes and effects, so it make commentary very long, hard to memorize, long time to learn, and confusion.  Summary of causes and effects is abhidhamma, that buddha taught to sariputta,  who buddha declared as "the best dhamma teacher (like him)", to let him know the whole causes and effects that possible have. 

Why buddha taught sariputta? Because sariputta ask him.

So most of abhidhamma is "Q&A" like commentary.

The history of buddhism, that we have learned from the western is not perfect enough. Many words that they overlook. It made them have wrong decisions and view.