Because he treats listeners as if they are smart.
That is the significance of "They want us to think" and "The strongest argument is" and "The arithmetic says one of three things must happen" and even "Now listen to me here, this is important." He is showing that he understands the many layers of logic and evidence and positioning and emotion that go into political discussion -- and, more important, he takes for granted that listeners can too.
The main other place you hear discussion based on the same assumption that people of any background, education level, or funny-sounding accent can understand sophisticated back-and-forth of argument and counter-claim is sports-talk radio. ("I understand the concern about Strasburg's arm. But ... ") You hear insults and disagreements and put-downs on sports-talk discussions. You rarely hear the kind of deliberate condescension, the unconcealable effort as if talking to slow learners, of many political "authorities" addressing the unwashed.
It's the difference between clarifying, and over-simplifying. Clarification, with the confidence that people can understand the back and forth, lies behind passages like this, which characterized most of the speech. Emphasis on the parts that show his approach being applied:
We Democrats, we think the country works better with a strong middle class, with real opportunities for poor folks to work their way into it, with a relentless focus on the future, with business and government actually working together to promote growth and broadly shared prosperity. You see, we believe that "We're all in this together" is a far better philosophy than "You're on your own."
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