Because Paul Graham, who founded Y Combinator and by extension Hacker News, loves Lisp. He used it as the primary language at Viaweb (basically late 90s Shopify) and viewed the language as a key enabled of their success.

Paul’s blog posts on the subject are useful background: http://paulgraham.com/lisp.html

You should probably start with this one: http://paulgraham.com/avg.html