I can't speak for the privacy scene but in my country it's pretty popular merely because of anonimity (which boils down to not having to use a phone number) and Discord-like server/groups. For porn and other NSFW content, it is pretty popular.

Telegram got its popularity because of piracy and having your chats on cloud. It was never intended to give privacy to user but due to WhatsApp breaches they started promoting telegram as a secured chat app which is a toatal joke till this day.

Telegram, while often hyped as high privacy/security got popular because it was/is fully featured and isn't Google or Facebook. That's it

It's less invasive, less annoying, and can do all the stuff like gifs and stickers. So it was very easy to get people onto compared to pretty much anything that was actually private or secure.

Once enough people started using it, it snowballed into its own monolith of bloat.

Honestly, UI and PC client experience.

I find the UI in signal a bit off putting. Telegram grabs you with their funky stickers, clean UI and dumb features. I alps hate that Signal won't bother copying the messages to a new client.. Like, I have a 1Gbps connection, surely we can copy my chat histories from my phone to my PC? Nope, gotta start fresh on every new client..

If they did less dumb shit like adding statuses, and put some more effort into making the UI nice, more people would use it.

And I get these are dumb reasons, but they're real none the less

Maybe because it offers public chats and channels? Something other apps lack.

Also the best desktop experience out of all apps I've tried.

I assumed the popularity was not in the privacy scene, but rather in general population, just because of usability. It is just a more usable alternative to Whatsapp or VKontakte. It is pretty much the default messaging platform for young people like Whatsapp is for older ones.

Telegram came out a year earlier in that signal, and because immediately popular amongst young people and drug dealers in Russia

It's popular with furries because of sticker support. Furries are an anchor population for the larger world of IT/etc. It was never really about privacy, or signal would have taken off.

Honestly it was mostly a Discord competitor if anything. One with FOSS clients for desktop and Android.

The private chat is baseline implementation just to tick a box rather than anything practically useful.

I think the big reason that nobody's mentioned yet is simply that they were earlier. Back when projects like Tox and Matrix were first starting to pop up, telegram was already fully formed. Signal didn't come until at least a year later and didn't have feature parity until several years later. Telegram by contrast was a much closer experience to WhatsApp and Messenger, making the transition much easier, particularly for low-tech knowledge users.

By lying aggressively.

Lying about being the first phone app with E2EE (they're not even close, by over a decade if we count J2ME apps) because Signal was called TextSecure back when telegram didn't even exist yet. Lying about their protocol, lying about their backup system (if you're using group chats or regular chats which are backed up they are visible to the admins and any other claim is a lie), bullshit propaganda against Signal, etc...

Oh and by the way, Signal has now finally launched usernames, so you don't have to share your phone number to use it anymore.