Mixer sites are banned from this forum. And altcoin projects don't have huge marketing budgets. You will find that signature campaigns on this forum require a large budget to keep active. And this budget gambling sites can only manage.
I agree with the points you have mentioned. But apart from those reasons, I guess another big reason is campaign managers. I feel like the campaign managers like to reach out to casinos more. I guess it is easy to convince the casinos to run their ad campaigns. Actually, the casinos are looking to promote their platform wherever they can. Even they place ads on porn websites. It feels like they are desperate to promote their casino.
But if you talk about other services like exchanges, altcoins, and other crypto projects, I don't think actively looking to promote their platform. The campaign managers have to reach them first and try to convince those platforms to run their campaigns on forums.