Running a forum means keeping a database up and running, ostensibly moderating out spam, tending to user requests, maintaining some additional level of security, etc. They may have simply not been seeing enough traffic / value on their own forum to warrant the time and expense to keep it up.
I've run just plain simple Wordpress blogs that I'm sure had far, far less traffic than anything Spitfire does, and the comments were constantly filled with spam. Keeping the Wordpress software and MySQL software and Apache software up to date, while not really that complicated of a task, can be an annoyance.
Telling their users to go chat on Reddit or VI-Control -- both of whom are in the core business of providing forums -- probably makes good sense. Spitfire can now spend that much more time improving Abbey Road Two Iconic Strings, and hopefully bringing us Abbey Road Two Iconic Small Brass Section!