That's because they refuse to make it seamless in a way iMessage is. Or Telegram. Both of those will work on several devices, have dedicated clients, synchronize messages and iMessage will transparently use SMS (on all platforms).

Signal does nothing of the sort - it's impossible to backup conversations, they won't sync to a new device, it won't work on a tablet, it's SMS support is buggy and incomplete and their desktop client is based on a dead technology (and limited to a single machine without SMS support). It also doesn't offer anything over builtin SMS/call support like Telegram does. To top it off, it hijacks SMS store meaning you can't even use OS-based software to backup at least SMS conversations.

I can't give Signal to non-tech users and just say "hey, use this instead of SMS", because it won't work and it'll lose their conversations. I can do so with iMessage.