I had the same issue, and I believe it's tied to UI/UX.

Signal, at least on Android, looks like a stock SMS text app. That turns "normal" people off because it looks and feels different from a "messaging" app like Slack/Messenger/Whatsapp etc where doing stuff like group chats, attaching video/audio/images is easy. When people see a standard SMS text interface, they feel it's for one-to-one communication only with limited multimedia capabilities (remember MMS?).