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?
Gamblers are more comfortable chatting in the internal chatting channel while playing because the username in the gambling site and on the chat channel are the same, and some gambling sites do not need outside channel this is to avoid confusion they can easily trace the account of the username on their platform.