It's not a matter of not supporting it; if you can store UTF-8 (as HN does) then you can store emojis. It's that HN actively strips emojis. One of the more childish and petty aspects of the site IMO.
Yeah. If you look at somewhat similar forums like Less Wrong, people don't even use emojis that often. So not a big loss on HN anyway. Markdown and LaTeX support however...