There was some ruling back in the 1960s (or something like that) that barred the NFL from having games on the same days as College or High School games. I think it had to do with their anti-trust exemption. Dos anyone have more details?
If you look at Wikipedia under the Sports Broadcasting Act of 1961 (http://en.wikipedia.org/wiki/Sports_Broadcasting_Act_of_1961) which references Title 15, Chapter 32 of the United States Code (http://www.law.cornell.edu/uscode/text/15/chapter-32) which defines "Telecasting of Professional Sports Contests" in dealing broadcating TV. Then to quote wikipedia
"It also, in effect, protects high school football and college football game attendance by blacking out pro football games locally on Friday evenings and Saturdays during those sports' regular seasons; these measures effectively outlawed the broadcasting (and, in practice, the playing) of NFL games on those days, since virtually all of the country is within 75 miles of at least one high school game on every Friday night in September and October."