It was a many months transition, and it's finally done
Fun thing, you can actually make a backup of all* your messages, groups, contacts, etc. So before leaving you can have all of your data in case you need that one contact or something
The final red flag was as that allegedly Russian authorities were messing with people's deleted messages. Not for the first time there are news that they could read, modify, delete, see location, and etc. Screw it, this is unsafe, I'm out.
Also, these days telegram is really at the state of a pile of garbage, bloated, buggy, and shady messenger.
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.