Always interesting when I hear people’s families use something other than the default in the US. What started that? Were your parents always concerned about privacy? Did you somehow convince them to use that app? What about when they text someone who isn’t on signal, which is probably 90% of the US? I could get my family to download the app but I feel like it’s pointless because no one else uses it so they would just quit.
The problem in the US is that the iPhone has a huge amount of the market and iMessage mostly does that already. For Android to counter that you have to use a 3rd party app, which further fractures an already smaller market. Or everyone could just use sms which is free with basically every plan nowadays, which gets you by.