Well, for one the key tenet of fascism is class collaboration, where all classes of society work together under 'the State'. Marxist class struggle, on the other hand, is the complete antithesis of this, formulating that the class interests of the capitalists and worker's are irreconcilable and can only be solved by revolution and the worker's taking over the State. The State then, for the Marxist, is only ever a tool of class rule, in this case a ruling working class.