It's not that the speaker or writer these days is less smart (although that might well be the case); it's the fact that the audience — the "people" — are dumb.
Meghan Daum's example that President Obama's State of the Union addresses "merited the lowest grade-level scores since FDR" actually proves my point. One doesn't graduate from Harvard Law School thinking or writing at a 7th-grade level. But someone who is that smart knows that he cannot communicate at that level because the people wouldn't understand him; any declarative statement that consists of more than five words and that doesn't end with a question mark simply cannot be understood.
Obama andsome of the other named politicians know this.