An instance only knows about communities it's been told about. lemmy.world knows about your community because someone there has searched for it, which indexed it and it now has 3 subscribers from there.
If it's giving a 404 error on other instances like .ml and blahaj, that means nobody from those instances has yet been the first to ssearch for it and those instances just straight-up don't know it exists yet. Note: people not registered there can't index new communities.
Makes sense, as your community is very small and I can't quite tell what it's about but it looks pretty niche so might not be something people are looking for in big numbers.
You are already listed on lemmyverse.net though, so if anyone is very interested in whatever your topic is they'll be able to find you and index you on their server easily. Other than waiting for that to natually happen, all you can really do is get the word out. Hang out in spaces that might be interested in your community and namedrop it as appropriate, post to the "hey look at my new community!" places, etc etc.
Or you could also make a ridiculous number of burner accounts to go around indexing it on ever server but that would be pretty pointless if nobody knows it exists in the first place!