Signal is pretty great and I use it at least a little myself, but it's not necessarily perfect. The big elephant that's always been in the room is its dependence on one backend and the fact they're based in the US.

Different threat models call for different approaches, and every option has tradeoffs.

• Dropping SMS support made sense from a security standpoint but slaughtered adoption
• When the central server eshittifies, we'll all have to switch again.

Because people stick with the default. There was a time people I know would try different messaging apps but after hangouts got canceled nobody wants to bother.

Because Signal requires a phone number.

Because Signal 1) doesn't ship as default, and 2) requires a login process.

You've already lost the majority of the general public with #1. The Venn diagram of people who go through with #2 and the people who already know about Signal is nearly a complete circle.

Because signal dropped SMS support.

I use Signal via Molly and as @lka1988@sh.itjust.works points out, there's friction in getting users to on to it. But also the UX isn't even as good as WhatsApp let alone Telegram.