Its the default in most of the world, there is no moving anyone to Signal since WA is just enough for most of the population and basically have the exact same features. Telegram has a bit of an audience but its only because its more like a social media/chat app hybrid.
A good chunk of the world also runs on low end phones and having an extra chat app to chat with maybe 1 person at best is a waste of space to most.
It got steam because it was one of the first ways to technically “chat for free” even if it had a price tag at first. Most countries had unlimited data but still billed people for every sms sent. Whatsapp introduced the concept of an app solely to chat with others through their phones for free/a one time fee forever.
By now it has reached critical mass and its basically impossible to get people to migrate anywhere else. Everyone would have to change apps at the same time.
People don’t really see any particular appeal in WA, its just the default now even if its just meh. Sadly.
In the US Apple had an early lead with the iPhone and so a lot of people converted from SMS to iMessage.
In the EU the iPhone didn’t have the same adoption, so when WhatsApp came along most people were still communicating with SMS, so it was WhatsApp which captured the market instead.
