Because autocrats desire centralized control, and Marxism delivers by insisting that the state should own the means of productions. Autocrats also like to pose as populists, and Marxism delivers there too. Even more conveniently, Marx himself said that full communism would take a long time to develop, which provides an endless series of excuses for underachievement. Also conveniently, Marxism contrasts itself explicitly to market capitalism, which provides autocrats in poor countries with an automatic enemy in the capitalist West to keep the masses enthralled.
All in all, if I were an autocrat, I’d probably find Marxism pretty congenial. Would I care about what Marx actually wrote or what his economic ideas really were? Not really. Every autocrat has his own national version of Marxism anyway.