It’s mostly open-source. Yes, the server is not, but that’s still better than a lot of others.
It’s ridiculously feature-rich (looking at you, Signal). E2EE chats for those who want it, JSON/HTML backups, API access, audio message transcription (for Premium users), actually cross-platform with native apps as opposed to browsers that load a terrible website (ahem, WhatsApp), bot support, and tons of customization.
It has a paid tier. This might be confusing, but having a paid tier is a good way to monetize an app that requires infrastructure at the scale of Telegram. It’s a little more assurance that they aren’t making money by selling your data (that said, this is not a guarantee).