it's one of the only commonly known messaging platform to most people that:

1: isn't owned by a company that many people hate, even if they don't know much about it.

2: isn't platform locked and doesn't discriminate.

3: doesn't require money or verification to use.

i can see how it's kinda settled out this way. whatsapp never caught on in the u.s. because everyone here was happy with sms and mms when the rest of the world was picking up Whatsapp. from what i understand that's literally just because texting was cheap in the u.s.. now people want more than mms, but apple is being apple about it so we need a third party app. by this point Whatsapp is owned by Facebook and the "privacy" of it is openly mocked by the average non tech person. everyone hates Facebook so the idea of willingly adopting another Facebook messenger that you're not already on seems crazy. anyone that would accept that is just going to use Facebook messenger instead. anyone that wouldn't will find something better.

Snapchat has no big controversies. no one knows who owns them. they don't really try to be more than a messaging platform. i can see why people would uncritically choose it as their default. i bet it's big among the kids who don't play that whole blue bubble iphone supremacy game, but don't use discord either. so i guess the non nerdy kids that aren't elitist dicks. that's who I'd guess uses it amongst the youth these days.

edit: before the suggestions come, I know these aren't good solutions. I'm just theorizing why. getting my friends on matrix as best i can...