I favor the overshadowed theory and Ryan's personality theory. I would add Carl Hubbell to the list on both accounts. He was a god in the 1930s with two MVP and leading the league in WHIP six times. And destroyed that killer line-up in the 1933 AS game:
Unfortunately for King Carl, his best years overlapped with Lefty Grove, who is alway on the short list for greatest pitcher of all time. Lefty led the AL in ERA 9 times to The Meal Ticket's 3, had a longer career, had a more productive career, and was just a better player.
Hub also was not colorful. Even though he played in NYC, he wasn't colorful. Grove was known as fiery, aggressive, a genuine red-ass in the clubhouse, and a power pitcher for much of his career (led the league in K's five times, to Carl's 1). Hubbell wasn't any of that. Americans love a colorful personality, even if it is an asshat personality.
Hadda throw in a few Wheaties from one of my favorite sets.