@tofm2 for one, DeltaChat is, if I got it right, basically server-less. Their network of mail servers provides for messages relay, but there is no storage there. DeltaChat is "pure E2EE", data is stored on clients. That's a huge difference vs Matrix.
As a consequence, I suspect there cannot be "public rooms" in DeltaChat, with content readable for newcomers. Same as WhatsApp groups, same reason: e2ee. Whereas Matrix allows for the "public room" type of chats/channels, by not requiring encryption and providing server-side storage.
@leanderlindahl @delta @matrix