|Norman Rockwell, illustration from 1917, "The Ungrateful Man," from the Google Art Project and the Norman Rockwell Museum|
Howard Pyle hit his first peak in 1900, but fell away after his death in 1911. He surged ahead in the 1920s, but I'm not sure why. Anybody know?
Norman Rockwell didn't enter the scene until around World War I. During his active career he was best known for painting 323 magazine covers for the Saturday Evening Post, ending that series in 1963. In all that time his renown never surpassed that of Maxfield Parrish. Rockwell's name was overshadowed by Pyle's until 1970, when Abrams published the book Norman Rockwell: Artist and Illustrator. The Norman Rockwell Museum started modestly in 1969, expanding to its current location in 1993, where it continues to build his reputation as his name became synonymous with small town life in America.
The names Howard Pyle, Maxfield Parrish, and Andrew Wyeth were mentioned about equally through the 1990s, but Dean Cornwell is not as well known. That makes it harder for museums and publishers to market books and exhibitions of his work.
Wikipedia--more about the Google NGram Viewer
The Norman Rockwell Museum