Terry Teachout has a theory that John Marin’s obscurity is a measure of American cultural inferiority but he’s willing to present the standard explanation for why the painter who was at his apogee at midcentury:

A bold colorist who viewed the American landscape through the kaleidoscopic prism of cubism, Mr. Marin conveyed with identical precision and sympathy the nervous angularity of lower Manhattan (“City Movement,” 1940) and the ceaseless turmoil of the waves that break on the coast of Maine (“Outer Sand Island, Maine,” 1936). Like all prolific artists, he was uneven in inspiration, but having seen dozens of his watercolors—he painted some 2,500 of them—I’m struck by how many are not just effective but indelibly memorable.

Why, then, is Mr. Marin so underappreciated by the art-world elite? The standard explanation is that even though he marched to the edge of abstraction, it seems never to have occurred to him to turn his back on the visible world. “The sea that I paint may not be the sea,” he wrote, “but it is a sea—not an abstraction.” After the rise of Abstract Expressionism in the 1940s, his deep-rooted belief in representation came to be seen as old-fashioned, even quaint.

How a Great American Painter Vanished from the Critical Scope (Wall Street Journal)