With the State of the Union address tonight, The Guardian plotted the Flesh-Kincaid grade levels for past addresses. Each circle represents a state of the union and is sized by the number of words used. Color is used to provide separation between presidents. For example, Obama's state of the union last year was around the eighth-grade level, and in contrast, James Madison's 1815 address had a reading level of 25.3.
My guess is this has to do with changes in how we write and talk more than anything else. Lee Drutman and Dan Drinkard for the Sunlight Foundation ran a more rigorous analysis on Congressional records back in May, and the declining trend is similar.