he could not explain how Magie’s invention, with its promotion of socialized land and shared wealth, had been transformed into the proprietary commodity that made billions of dollars for Parker Brothers.

Magie held the patent until 1935, when she sold it to Parker Brothers for $500. The company had recently started distributing Charles Darrow's Monopoly, and was buying up the patents of various commercial forms of the game in order to claim undisputed rights to selling it.

Wikipedia

would that explain it?