You more-or-less answered your own question: Mussolini's government did it. Draining swamps is a large undertaking. Even the US couldn't even get it right in Florida for 150 years or so, until we started trying to preserve the Everglades instead--there were too many competing interests. A literally fascist government is, for better or worse, more likely to get things done than the governments of present-day Africa, and you have to consider that the Pontine Marshes still need drainage maintenance today. Additionally, this site lists some of the positive sides of these swamps--they're not just malarial havens.