From a brief search, most presidential candidates speak at around an 8th grade level. (Which makes sense. They need to appeal to a broad audience, and speaking at a higher level is usually counterproductive anyway unless you have a specialized audience.) Trump's speaking level is more like 3rd or 4th grade.

From a recent Fresh Air interview with Trevor Noah. Earlier on he explains with a more personal anecdote why simpler language matters. But is there such a thing as too simple? Is 8th grade language more effective than 4th grade language? Later on Trevor Noah says

  You know, funny enough, one of the biggest moments of
  realization was when Donald Trump won the election because
  when I came into the show, I said, I think this guy can win.
  This was when he first came down that escalator. He gave his
  first speech.

  And then I was like, wow, this guy's going to do well. And I
  remember people laughed at me. People were like, oh, you
  silly ignorant person who's just come to this world. You
  clearly shouldn't be at "The Daily Show" 'cause you don't
  know what you're talking about. And I was like, but I don't
  know. He seems like he connects with people. I can relate to
  him as a performer. I can see what tools he's using. He's
  good at riffing. He's good at taking the crowd on a journey.
  I can see what he's doing.

  And people would say - and all throughout the race - and
  there were times when, on the show, I would mention it. You
  know, I mean, that's why I said Trump reminds me of an
  African dictator. And that's where that came from because
  everyone said to me, this guy is - he's just a fool. He's
  just - he's a buffoon. I said, yeah, you can say that. But
  I've seen this before. I have seen this before. I've seen
  clowns that go on to take over their countries. I've seen
  buffoons who end up ruling their worlds. And it came to
  pass.

  Source: http://www.npr.org/2016/12/16/505847054/trevor-noah-says-he-grew-up-in-the-shadow-of-a-giant-his-mom

As does most of the country...

Most of the country aren't presidents.

Presidents speak to most of the country