While Telegram and Discord seem equally popular by the number of users, it's not hard to see that Telegram users are largely people from former Soviet Union (and there's also a strong Indian community and Arabic-speaking community, I guess). These don't happen to be the places with the most active gamblers. Discord, on the other hand, is most popular in the US with Western European countries being close runner-ups. So the languages and the geography is quite different. Apart from this, Telegram tends to be a platform for personal communications and for politics (like the Belarusian protests relying largely on Telegram), while Discord, as has been pointed out, is a gamers' community. So I guess gambling is naturally closer to Discord than to Telegram by areas, users and usage.