It’s nowhere near as convenient as Telegram, the clients on each OS are at best OK.
Show me conversation history when I login to Signal desktop. Oh, yea, it doesn’t do that. Is there even a web login?
From a typical user standpoint, Signal looks like text messaging, and that matters to them. And it’s not simple to manage.
Signal is coming along, and I look forward to when it’s mature enough that I can get people to use it (again). But they also made a massive mistake in dropping SMS support. That was my best tool to get people to switch. When they dropped SMS, those folks all left signal.