Because Marx himself wrote that communism is a necessary form and a dynamic principle of the immediate future, but communism is not as such, neither the aim of human development, nor the aspect of human society, communists will most probably say that he is a fascist as well.
But he insisted, since as he wrote with Engels this time around, that communism doesn’t constitute a state of affairs which ought to be institutionalized, nor an ideal to which reality ought to conform to.
The only thing which saves Marx is probably the inadequate reading of his work, or else everyone would have known that it could not function, and they would have spoken ill of him.
Just as well they did not know what he was talking about.