As U.S.-Cuban relations warmed in recent years, Matilde Portela, an Airbnb host, reveled in the flood of American tourists arriving. An aspiring business executive at age 73, she quickly learned the art of niche marketing – adorning her home with two American flags and laying out back copies of the New Yorker. “We had so many Americans coming that we didn’t know where to put them,” Portela said. But Cuba’s Great American Tourist boom has now turned to bust. During the first three months of this year, 95,520 Americans came to Cuba – a 40 percent drop from the same...