I think it's mainly Telegram's native feel (and it is native on every platform it supports afaik). It's even in little trivial things like the rubber band effect on Apple's platforms, then in how smooth the loading of missing stuff from the network is, and finally it's in the design: Telegram is slick.

All those little things combined and when you switch from Telegram to Signal or WhatsApp it feels like going a couple of decades back, or something like that.

Honestly I don't know how much I can trust Telegram and its founder Pavel Durov (I probably shouldn't), but in terms of the quality of software it's unmatched.