Why do some doctors not know how to switch from medical jargon to plain speaking?

Sometimes a two word phrase in jargon would take a whole book in plain English to explain. What you're really asking is for the doctor to guess how accurate his information should be to suit your needs and desires (and your needs may not match your wishes!). It isn't a surprise that some can "read" what you're looking for better than others.

Just like Blehhh said, simple can be hard for some while some of the simplest tasks can be hard for others. They spent 10+ years in medical school and probably including their residency writing many papers and having to talk in that kind of jargon in order to understand the material and pass and even after school they still use those medical terms while talking to their colleagues about the patients condition, so it just becomes wired into their brains just like how we were taught to talk and pronounce words at a younger age by our parents.