I'm only guessing...
HTTPS is the new HTPP. Once everyone has it, it will be the norm.
HTTPS usually costs extra on a hosting package; this is a user-to-user forum graciously hosted by X10, so why would X10 spend extra cash on a service that's free to use?
HTTP is fine for the purpose unless you're a member of the Google Dictatorship who now flags HTTP as 'NOT-SECURE'.
Why would a forum where everyone (hopefully) remains semi-anonymous need a security layer?