A number of reasons. Just like you claim a Communist party almost destroyed your country, Capitalist parties destroy and are destroying many countries as well. The existence of bad Communist parties does not itself mean Communism is structurally a bad thing, as pursuit of a Stateless, Classless, Moneyless society is a noble goal for humanity.
I think it’s fair to say that decentralization is a good check against Authoritarianism, and as such, this should be extended to the workplace, not just government.
As far as why Lemmy leans left, the founder is a Communist, and principles of decentralization and federation tend to appeal far more to leftists, while Capitalist-inclined individuals have Reddit.