Comment by tambre

The most touted reason is that their anti-spam systems only support IPv4. Their old Cloudflare endpoint however is still alive and you can't disable IPv6 on Cloudflare so feel free to add the following to your /etc/hosts:

    2606:4700::6810:686e news.ycombinator.com

Interestingly when I tried to post the above comment over IPv6 I got a Cloudflare "You have been blocked" page. This might be something they do not want you to know! :D