It sounds like you need to do more research. Instead of asking there aren't more big yet serious communities like Hacker News, you should try to (1) ascertain t

It sounds like you need to do more research. Instead of asking there aren't more big yet serious communities like Hacker News, you should try to (1) ascertain the size of the HN community, (2) look into what other "serious" (whatever that means) communities exist on the internet, and (3) compare their sizes. If after all that you find that HN actually is the only serious community of its size, then you should re-ask this question. But as it stands your question might have no basis at all.