He was installed by a military coup, not a popular revolution, and then enacted some positive reforms but fundamentally failed to produce anything remotely like a dictatorship of the proletariat, and was thereby ousted by the military who held the power the whole time. Sankara was genuinely benevolent in his intentions, but his approach to revolution was anti-Marxist and it created a warped state that was basically a military dictatorship when it decided to be (though he did not support such a thing, he ultimately did not stop it), because popular support functioned as a convenience rather than the actual mechanism of decision-making.