Is it because bitcoin projects can't be easily monetized?
Is it because even if they were monetized, they couldn't have the profit margins that casinos have, so they could be efficiently advertised?
Or is it because only mixers could bring money to the campaingers? I highly doubt that banning mixers was such a powerful measure, that lead to the "all-in" from signature campaings...
Gambling is hugely profitable so that's why they advertise here as they're one of the only industries that probably see signature campaigns as worth it. For others it's probably not profitable or they don't have the budgets to throw around on such advertising. The are a few exchanges that advertise though and they are probably the second biggest advertisers here.