I don't understand

It seems the vocabulary levels of presidential candidates have declined significantly over time. Check this:

The Princeton Review, best known for its test preparation services, analyzed the vocabulary used by the presidential candidates in the campaign debates of 2000 and compared it to the vocabulary levels used in earlier campaign debates.

The Princeton Review obtained transcripts of the Gore-Bush debates, the Clinton-Bush-Perot debate of 1992, the Kennedy-Nixon debate of 1960, and the Lincoln-Douglas debate of 1858. It analyzed these transcripts using a standard vocabulary test that indicates the minimum educational level needed for a reader to understand a document. This test is ordinarily used to evaluate textbooks and other educational materials.

The results? In the debates of 2000, George W. Bush spoke at a sixth-grade level (6.7); Al Gore spoke at a high seventh-grade level (7.9). In 1992, challenger Bill Clinton scored in the seventh grade (7.6), President George Bush in the sixth grade (6.8), and Ross Perot at a sixth-grade level (6.3).

Our contemporary politicians, who found it necessary to speak to us as sixth and seventh graders, compared unfavorably with Kennedy and Nixon, both of whom spoke in a vocabulary appropriate for tenth graders. And they, in turn, looked sophomoric when compared to Abraham Lincoln and Stephen Douglas, whose scores, respectively, were 11.2 and 12.0.

The above quote is from a 2001 article by Diane Ravitch. Link. Ravitch asks, "is it the candidates who have dumbed down their appeals or are they simply acknowledging that the public has a limited vocabulary?" Ravitch argues for the latter, and her thesis is that this limited quality of Presidential debates is based on the declining quality of our educational system.