I don't have a medical background, but my suspicion is the process of formulating a thought, deciding what meaning you want to convey, and then settling on what words accomplish that is far more complicated and delicate than we give it credit for. I suspect any sort of issue can wreck the whole process, which might explain why really good communicators are so rare.

For myself, ADHD medication really helps slow my brain down and thinking things through before opening my mouth. Turns out that's an important part in (properly) verbalizing thoughts.