Well signal is designed for secure communications for most people. Not designed for anonymous communication. So a key piece of that is easy to use, nice on boarding process, and being useful to new users in that first minute or two.

So the reason signal is so useful is because so many people use it, which depends on things like contacts showing up immediately when you join.

Seems like it's a common theme that people want a messaging client with a huge number of users, but don't want the features that made it popular in the first place.