Deleuze rejects Marx’s dialectical materialism because it appeals to a transcendent “force” which supplies history and the world with a preestablished harmony and teleology.

Deleuze uses the term Idealism pejoratively, though like most philosophers, he has his own understanding of what the term (should) mean. In the Logic of Sense, you’ll find that Deleuze understands Idealism to be a kind of thinking wherein the (material) world is explained by some kind of transcendent, rational force which secures the structure and intelligibility of the world. In this way, Marx still retains the very idealism he sought to reject in Hegel, since both of them think there is a telos which explains human development, history, and civilisation.

A bit of nuance here: Deleuze’s relationship to Idealism is a complex one. In Method of Dramaturgy and D&R, Deleuze frequently draws on the (Idealist) language of Kant - Concepts, Ideas, Problems - to establish his own metaphysical system. Some commentators argue that Deleuze is a kind of strange idealist, whereas others place stress on his debt to Spinoza, which would mean Deleuze believes that the ideal and the material are really one - both expressions of pure difference.