E2E is not available on all platforms, is hidden in obscure menus and the whole UI discourages users from using it. Telegram is a data-harvesting social goolag-oriented network after all. :-/
The greatest feature that telegram offers is cloud sync. Everybody knows the limitations E2EE comes with. There's no way you could have thousands of members in a group on Signal.
Along with that, the ability to manage device sessions and to login on multiple devices with full chat sync is extremely unique to Telegram.
You're asking them to ditch that in favor of inferior UX, which they simply cannot do at this point.
But I do hear the valid complaints. I do believe they should improve MTProto 2.0 to work on multiple devices and in groups. Their implementation is fine for 1-1 chats but having something better than that is always welcome.
