All three of the examples you laid out have specific use cases that propelled their popularity

WhatsApp: free SMS which was a bigger deal, especially internationally when it hit the scene. The Echo Show is US only for now.

FB Messenger: access to the walled garden of people that live in FB

Signal: secure private communications.

I'm not saying that Apple users won't use other message mediums if there is a compelling reason but I am saying for Apple users that are in predominately Apple-centric family/social circles the Echo messaging/video chat app doesn't offer meaningful differentiation from iMessage/FaceTime.