Why block Telegram and not .. all the others? Whatsapp, Signal, Viber, ... Are they objecting to the use of encryption? Because everyone uses encryption almost all the time for everything everywhere. The cat has been well and truly released from that bag. Are they just going to turn off the internet altogether?
The FSB wants encryption keys. Telegram won't share, so is blocked. Therefore, Whatsapp and Viber gave up their keys, I suppose. Or provided backdoors.
Telegram's independence is what separates them from other services, what allowed them to deny the demand for crypto keys, and why they were targeted.