Features/ease of use mainly. It's very easy to get on a Telegram group, share links to content on it both on the internet via links and also to other Telegram channels. It handles video, images and audio exceptionally well. And I think probably the biggest reason is that it doesn't make you have to know about encryption stuff. As for security, Telegram was listed as the top in difficulty for the glowniggers to get at your data (not talking about channels, though, only the encrypted direct chats with auto-delete). For those of you who won't use it due to the phone number signup, they added a way to get a unique anonymous phone number via some cryptocurrency (you don't need to link any real world identity info to it). Last time I checked a couple weeks ago it would cost about $15 worth to set it up. Personally I just used a Joogle Voice number. All that said, I like Matrix. I lost my login to Poal Chat though :(