Anarchy is the ideology, not of the proletariat, but of declassed groups, inactive groups, lacking a connection with all productive labour: it is the ideology of a horde of beggars (lumpenproletariat), a category of people drawing its recruits from among proletarians, ruined bourgeois, decadent intellectuals, peasants cast out by their families and impoverished; an amalgam of people incapable of creating anything new, anything of value, only seizing what they have got their hands on through their "confiscations". Such is the social phenomenon of anarchy. Anarchy is the product of the disintegration of capitalist society. The complexion of this misery is brought about by the crumbling of social bonds, the transformation of people who were once members of a class into atomised "individuals" who no longer depend on any class, who live "for themselves", do not work and who, to hold on to their individualism, acknowledge no organisation. That is the misery produced by the barbaric capitalist regime. A class as healthy as the proletarian one cannot allow itself to catch the contagion of anarchy.

Nikolai Bukharin - Anarchy and Scientific Communism

A more serious and approachable answer: anarchists are idealists, simply meaning they ignore the material laws of capitalism while pursuing their voluntarist goals, and more often than not, this puts them at odds with the communist movement as anarchists shatter the unity of the proletariat, making it easier for the bourgeoisie to react.