Probably because "join Lemmy" is way too broad and Lemmy sites don't have good page rank scores yet. With any luck, the sites becoming bigger will improve results.

That said, I'm not sure if the page rank algorithm will even work very well on them without changes, since there's so many possible URLs for different instances view of the same post. It likely dilutes the rank of any particular result.

As an aside, even if Google worked well, I suspect it would end up causing the biggest instances to only get bigger. Google gives weight to what other people link to, and that's most likely to be from the largest instances. Also, the whole idea of creating new instances probably doesn't mesh well with how web search works, since it isn't likely to trust new, unfamiliar sites.