I think it's a simple matter of focus. One could extend your logic and say 'Why don't you get your tech news from NY Times instead of Hacker News, since they have a tech section in their website ?'
True, but making a NY Times-like website for tech news wouldn't work either. HN works for techies because its clean and simple, and the karma/upvote mechanism keep the content relevant. So there is a difference from NY Times.
I never said it claims to be better and it isn't. Let me rephrase the question: why would anyone want to be on echojs instead of HN, given that it's essentially an HN clone for JS and HN community is bigger at the moment? Why would you use it?
I am just trying to understand its value for myself.