Signal is common in Western governments, particularly the more sensitive and diligent parts. If you work with government people, and I do, that tends to be the messaging app of choice and sometimes policy. WhatsApp also tends to be commonly used in Europe for official government things despite being forbidden for such purposes, but that isn’t seriously enforced. If you wanted an intelligence backdoor into the entire European political apparatus, WhatsApp is the way to do it. Colloquially and by inference, Signal seems to be considered the most secure common messaging app in governments, though this doesn’t mean it is actually true.
I’ve used a lot of messaging apps around the world but never had a need to install Telegram. I know it is used in Eastern Europe but I never had a need to install it even when I was there.
I’m just a user but my messaging footprint is more global than most.
I think this is really why it never caught on here (having fallback to SMS by default). I have an unlimited voice and data plan here in Spain, and don't get any SMS included at all. There's not even an option with my provider to have any included. 0.1 euros per SMS, it's kinda crazy. I maybe send 5 or 10 per year, usually some kind of response to an automated 2fa system or etc. If iMessage ever took off I'd always have to be aware of the green/blue bubble thing and make sure to never send a message using SMS. WhatsApp/Telegram/Signal/etc is just easier since you don't have to think about it.
BTW, what about Telegram? It seems like it's pretty common in Eastern Europe.