The absolute best book any student of the rules can buy is "Anatomy of a Game" by David Nelson. It was published in 1994 and was year by year analysis of the rules and changes starting in the 1870's.
He reported that in 1930 the NFHS decided to write its own code after being denied a seat on the NCAA Rules Committee. The game as very dangerous at that time. There were almost 50 deaths a year, with 80% being at the HS level. But Texas did not join the NFHS until 1969 so for 40 years I suppose the reason was that Texas was not part of the NFHS. By then, Texas HS football ( and the coaches) became so powerful it would be near impossible to get a change , and that is true still. For many many years, Texas high schools had a rep in the committee room , even if not officially on the committee, so there was widespread belief Texas HS concerns were being addressed for the most part.