In communication, you must talk to your audience. You want the words to be common and the grammar to be simple in order to reach the widest number of people.
I am a scientist. I could say, “Two aliquots of 1.5 mg of N. meningitidis antigen suspended in phosphate buffer injected intramuscularly at intervals of two weeks induced a protective antibody titer of 1:500 (95% CI 1:433 to 1:564) in 457 out of 498, or 91.8%, of study participants.” Or I could say, “Two shots of meningitis vaccine protected over 90% of the test subjects” and reach a lot more people with my message.
In my experience, many people who use complicated jargon-filled language to non-expert (but otherwise intelligent) audiences do so not to communicate a specific idea, but to show off how smart they are. As far as I’m concerned, anyone who knows their stuff should be able to explain it in language that school children can understand.
Trump obviously has the intellect—a dummy could NOT have achieved all that he has. But he’ll save the esoteric language for his professionals, and use the ordinary language to communicate his ideas to the public. Truly intelligent people can do that.
I like your comparison of a sentence filled with scientific terms with a sentence that simply makes the point. That explains much of why Trump chooses simple language, but I think there is another part.
The last several decades has seen a marked loss of critical thinking skills and the language itself reflects this change. Today’s speech is filled with made up words and fallacial thought processes that intentionally avoid clarity. Clarity is often uncomfortable for liberals seeking a safe space. Politically correct jargon is designed to obscure conclusions, such as what is true or false, right or wrong, winner or loser - all of these dichotomies are discouraged and the very words that define them are purged from the language and replaced with psycho babble and newspeak.
So, I agree Trump’s simple speech is partly to connect with the average person who has no need to bother with more advanced vocabulary. However, I think he also realizes that he must avoid the fancy sounding sociology economic language used by politicians, social engineers, community organizers, academics and journalists, if he is to cut through the politically correct layers of agenda filled nonsense and speak the truth.