Signal may not be the best in a technical sense, but it is good enough and it has the network effect. I’ve been pleasantly surprised when in the span of a few months I met two different people actually in real life, who happened to already be using Signal.
Signal is also just as usable as the big tech alternatives, which makes it not a very hard sell to friends and family. For quite a few years now I have managed to convince everyone I communicate with to do so over Signal. There is no chance I would be as successful with something else.
I was using Ubuntu Touch back when Canonical launched it in partnership with BQ. I still remember how annoyed I was when reddit kept telling me to download the app or continue with “Google Chrome” when I was on Firefox. Those where the days.