Because reddit still has a huge userbase compared to Lemmy and that brings content, engagement and revenue, they are an institution of the internet at this point. Reddit posts are part of google results while Lemmy does not, when people have a problem they find old reddit threads for help, guides and tech support, not so with Lemmy. I would say 95% of reddit userbase doesn't even know that Lemmy exists. One fuck up will not kill reddit as it currently is, they are too massive, one fuck up might kill Lemmy, if it just doesn't slowly waste away. Reddit would have to fuck up constantly over a long period of time, kill communities, put features behind paywall, get caught in spying of the users, etc. And each time Lemmy would have to be advertizing itself in every twist and turn to get those users and not alienate them and be able to support the growing userbase and gain some benefit from them and them not just be a cost sink of lurkers.