It's not Lemmy it's mostly lemmy.ml and lemmygrad, two of the biggest instances. As both name's imply both are communist in nature but both host communities for multiple topics since they where the most active instances. The Lemmy Devs originally created Lemmy due to Reddit banning/quarantining multiple leftist subreddits around 2018/2019 and created lemmy.ml as the first instance. Don't know the history behind lemmygrad but it's a communist instance nonetheless.