We do not have the luxury of restricting ourselves based on whether the platform is using proprietary software or not, we go where the potential contributors are. One could be forgiven looking as the software choices we use, and are getting engagement on, that we are bias towards FLOSS platforms but what you are seeing is "Effect" and not the "Cause". Starting out our policy was to use existing platforms as much as possible, we were not afraid to use and pay for proprietary software. However in our nearly 19 years of operation we saw that companies come and go. With proprietary software you are left high and dry while, with FLOSS software you can find someone to host it. 

The show focused on our IRC channel, and while it's true we did have a thriving community there for a while, that stopped when Freenode got taken over. We maintain a presence on IRC, but most people have moved to Matrix where the contributions and feedback has been very good.

You suggested (possibly ironically) replacing IRC with a BBS. From 2005 each show was automatically posted for discussion there. Unfortunately that community was abandoned when the corporation producing the software stopped support. It was shut down after it was compromised, and then the site got over run with spam.

Our comment system was a third party application running on cPanel. The corporation producing the commenting software stopped support, and after it was compromised the site got over run with spam. For a time the entire HPR domain was blocked as a spam site. Dave and I were able to salvage the database, and put in very robust checks on code that is now released undo AGPLv3.0. Since then cPanel upped their prices to such an extent that we migrated to a static site also available under the AGPL, but we were able to just move the MailMan operation to a service which we pay for.

In 2011 we jumped on the Google+ bandwagon and did get a lot of contributions, until it was shut down in 2019.