...Most of them are from Chipmixer and Bestchange's campaigns, and some of them are from the rest of the campaigns?
These campaigns force members to create 50 or 25 topics posts per week, so the frequent posting will make them get merits regardless of the topic.
ChipMixer has no minimum post requirements. In theory, you would get paid for writing only one post per week, but that's surely not something that works in their favor. 50 is the upper limit, you are right about that. BestChange pays up to 25 posts weekly, and if I remember correctly from my brief spell in that campaign, there is a minimum requirement of 10 posts.
But other campaigns have a 25 or 20 posts
minimum requirement meaning you need to fulfill that to get paid. Hence, that can't be the reason why most or why the best replies are from CM and BC members.