I want to make it clear that this answer is pure speculation, and I have several ideas:

1) Many individuals with ADHD, even when they appreciate the benefits of their medication, simply forget to take them on a regular basis, because of the very inattention of ADHD.

2) Many individuals with ADHD don't like taking medications, even if they know that they help them function better. They feel it dampens their spontaneity, creativity, or individuality. They feel it might make them bland or boring.

3) Some individuals with ADHD feel that acknowledging their ADHD is an admission of a "defect". Some admit to have ADHD, but see taking medication as a crutch, or weakness, or indication that they are deeply flawed. For someone with strong narcissistic traits and ADHD, it is often much harder to acknowledge the benefit from daily medications.

4) Trump has created an echo chamber of followers, who like him when he is loud, rude, disruptive, and defying conventional standards. In settings when he is with admirers who have those expectations, being organized and thoughtful is actually a detriment. His people view pausing to think before one speaks as being scripted and deceitful, with the implication that one must be hiding what one really feels.

5) Being unpredictable may serve to keep enemies, both domestic and foreign, on guard and uncertain about how to proceed. Some of this is similar to older notions during the Cold War, about a ruler of a small nuclear-armed state might deter attacks by acting like a madman.