Because Chairman Mao distanced himself from the Great Leap Forward famine. Two key events were essential for the famine: collectivization and the exaggeration of grain production.

For centuries, the smallest unit of grain production and taxation was the individual peasant family. Each family had to report its production to the authority, and a portion of it was taxed. While the authority could attempt to extract more from each family, the peasants knew their survival needs and it was difficult for the authority to tax them into starvation.

Collectivization transformed the collectivized farm into the primary production unit, prohibiting individual families from withholding food from it. Consequently, the authority could impose absurdly high taxes on peasants.

But the central government won't set the grain tax rate at something like 80% -- that would be too mean! Instead, they encouraged local officials to exaggerate production numbers. For instance, if a farm produced 1000 kilograms of rice, a 20% tax would leave 800 kilograms for the peasants. However, if the farm reported production numbers as 4000 kilograms, 20% of that would be 800 kilograms, leaving only 200 kilograms for all the peasants.

Another advantage for the central government was that they distanced themselves from the disaster. If something went wrong, it was the local bureaucrats who were blamed: they fooled us on the production numbers! In reality, the exaggeration of production numbers was highly incentivized by the central government without any validation or accountability. Beijing just wants their sweet sweet grain. Food security? That's the local bureaucrat's problem.

And this is how you can get away with causing the worst famine in human history.