Reddit was big before the Digg migration and got bigger still. It didn't happen overnight, it took many years. Reddit also benefited from celebrities and other influencers using it to become the default site for this type of content. Lemmy's problem is there's no void to fill, Reddit took a hit from the API fiasco but it's still going strong because 99% of the users didn't care, or returned soon after. Every subreddit I was in that chose to close down has returned to normal operation, and it's not even 2 months later.
I like Lemmy, I'm going to keep coming here to see how it grows. Right now, it's not even close to being a Reddit alternative. It's barely hanging on, but I wish it the best.
There's a few third-party apps that have been granted exemptions from the API pricing changes, but other than that the majority of users are using the official reddit app or "new" reddit website because they don't know any better.
There's plenty of newer Reddit users that got in when the official mobile app and the new theme was default. They got used to it and never cared about the death of third party apps or the eventual downfall of old.reddit.