Bitvest and 777coin has more participants and more paid posts compared to other signature campaign that is why you are seeing most of their post but it doesn't mean they are the only gambling campaigns were running.Bustadice,yolodice,bitcasino and lot other campaigns are also active for years now.
Not only because they have many participants , they are always on the top because there is always available spot on their campaign unlike other long running campaign that always full and no vacant spot for other participants and only bumping once a week when they send the payment .
OP doesn't notice other thread because no one is applying and its always full.