Authors back then made nowhere near the money they make now. All the books he had published went to support all the families, plus his own habits. And that probably came to very little income for him.
And he was writing in the years of Steinbeck, Pearl Buck, Hemingway for the Americans and for the Brits, Agatha Chrristie, George Orwell, Virginia Woolfe, etc. The huge signing commissions weren't given to guys penning pop fiction and they didn't sell for top dollar.
Today there are movie rights, TV rights, digital media rights and other sources of income for a big writer. No surprise all his money evaporated.