In the UK I don't know a person without WhatsApp, pretty much everyone uses it. I don't know a single person who has signal.

If I say to a friend "I'll message you on WhatsApp" they will be like "ok great".

If I say to a friend "I'll message you on Signal" they will be like "What is that? Oh I have to download something? Don't you have WhatsApp? Screw it just send me a text."

So you can't really say "Apart from larger user base why do people use it?". It's like saying "Apart from an inability to breathe, why don't people live underwater?"