The data behind phonics is heavily tied up with America's wars.
That is because wars are when the gov't gets its hands on large numbers of post-HS men and pokes, prods, and tests them.
Over the course of some of our larger wars, say, from the Civil War onwards, the writing ability of Joe Average Soldier has declined (controlled for education level). That decline was accompanied by a shift from phonics techniques to others (whole word, etc.). So, there is a good amount of correlation, but that sort of data can not be used to determine causation.
Which brings us to Education Colleges/Schools. Back in the day, they were used to train teachers to teach, using methods that had worked and been found to work. Despite any lack of sophistication we might ascribe to such institutions in the 19th Century, such schools had the advantage of dealing with a known factor: human nature. It had not changed since Shakespeare or Homer. What worked for a majority of Athenians, pedagogically-speaking, will work for Americans. When it comes down to it, you don't need computers, letters behind your name (BA/MA/PhD), or an understanding of genetics to teach children to read.
Fast forward a bit and these Colleges of Education get mroe "sophisticated." PhD programs develop, along with the need to publish/propound new & different theories, techniques, etc. An ambitious young man or woman is not going to get that advanced sheepskin if he boils down his thesis to: "Anyone of average intelligence and education can teach a child to read, if they are of a mind to do so."
Instead, you see a baker's dozen of educanto theories proposed and inflicted upon kiddos. Add to it that this was during the ascendancy of the Progressive Movement, which had(s) as goals the indoctrination of children, alienation of them from thier family, and had a great prejudice in favor of technocracy. And saw individual humans as mere cogs in the machinery of the State.
