I haven't seen any Gamblig Platform who used Telegram for Promotions, most I know is that they use Discord.
Do you have any Idea why they don't have any Telegram Communities? and why does it seem they don't use this platform for their promotions?
I don't know why most casinos prefer Discord to Telegram, but for the big Poker Casino I know that it's much easier for them to provide support for players. It's similar to the live chat help on casino website, but they can pin and highlight messages much easier. With Discord there is the way to make many different sub channels. So managing a lot of different users is fairly easy on discord with only 1-2 community managers. I am not sure about telegram and how you can manage a lot of people there.