Mostly, the platforms you are talking about are used by projects and platforms to promote their services using ads, but as far as I know, Facebook mostly doesn't allow casinos to advertise themselves, but I could be wrong. This could be a reason why many casinos aren't present there. I don't say that BetHog shouldn't have a presence there, but they are probably targeting markets where they can have more exposure for the business they are running.
You are right. For now I think only Telegram and X give the freedom to promote something that is 18+ or something that is considered illegal in some countries and legal in others. I don't think BetHog should try to create Instagram or Facebook if they don't give permission for BetHog site advertising. And actually we can see some other platforms that are successful just by using Telegram and X. As @Alphakilo said, a platform needs a promotion and marketing team that can create good promotional content for certain events.