The difference was Reddit had already built up a reasonably comparable audience when Digg imploded so the migration was easy. If you look at a similar graph of Reddit today and Lemmy/Kbin, you probably wouldn't even see these tools register with the active user base of Reddit so high. I think "rhyme" of history is that another service will eventually win, and it might be ours, but it's more akin to the fall of the British Empire than an overnight event.