I think a lot of the reason why trump's speech is so low-level most of the time, is because we see him addressing the public - he's very good at leveling with his audience, and as we know - most of the public are rather simple.

In interviews he is very careful with his words - like in the Peirs Morgan interview where he only offers to apologize to the UK for sharing a racial hate video originating from the UK; rather than actually apologizing. He might not use big words, but he uses the right words in the right way more than you might think.