There was a very similar question asked few months ago. The reason is that while fascism professes nationalist authoritarianism which by definition resulted in oppression, war, and the mass extermination of multiple ethno-religious groups; there is nothing intrinsically authoritarian about the writings of Marx, Engels, or any of the 19th century thinkers traditionally associated with Communism. This is particularly evident when juxtaposing the two ideologies in my own area of interest: Italy.

Although the actions of leaders acting under the pretext of fulfilling the promises of Communism are certainly authoritarian, the term is sufficiently broad that it can take multiple meanings, many of which are markedly anti-authoritarian. For example, you might be interested in the various questions I answered on the Italian Communist Party, which was the most effective opposition party in Italy between 1947 and 1989: I answered this question about left-wing parties in Italy which might give you a good overview, as well as this other question about the political philosophy of the later Italian Communist Party leadership.