@nprofile1q... afaik it is not a blanket policy. At some point the Discourse was set up, and over time more discussion happens there. So especially in areas with already low volume of discussion, having two different channels can split already small participation. (e.g. talk-nz mail vs Oceania section in the forum)

Here's some background info
https://wiki.openstreetmap.org/wiki/Mailing_lists