[forshaper]

The software is never the issue with this, it's where people are that's the problem. Though I did witness my age-peer friend groups finally switching to Signal in the late 2010s (away from Facebook Messenger), I don't actually know what convinced them. The security-conscious minority element had been pushing it since it started but were generally mocked. I think it finally showed up in a New York Times article, which is what helped them.

[Pay08]

I'm curious why WhatsApp isn't much more popular in the USA. Is it the lack of anonymity?

[Cider9986]

No, I don't see any indication it has anything to do with the "anonymity". Very few people, even technical people care about anonymity to the extent that they try to achieve it in everyday life.

It is frequently confused with privacy, however. (https://www.privacyguides.org/en/basics/why-privacy-matters/)

iMessage is the dominant messenger because most people have iPhones combined with the fact that SMS has long been free and unlimited, so people don't see the problem of using it with the occasional Android user.

Really, it's all about the defaults. Even though everyone uses iPhones, they still use the calling feature from their cellular provider, because Apple doesn't push FaceTime as the default calling mechanism.

Signal is gaining popularity because there are people that care about using it over iMessage.

Signal is 100x better than WhatsApp, but it feels so unstable using any centralized messenger that has complete control over the software and the users. No centralized service can truly be relied on, non-profit or for-profit. But clearly that's what has to happen in order for the service to become mainstream, so it's an acceptable compromise for me. It's not like I can't say Signal does great things for privsec and metadata reduction.