Relatively low amount of member slots in comparison to other campaigns and lower pay too.
In fact, the payment rate for Blender.io campaign is much higher than Playbetr campaign, which I participated in ($3 per post compare to $2 per post). I think you missed important point, that is one of reasons why people applied and tried moving from their current campaigns to Blender.io.
This isn't a general reduction on the number of weekly posts to be made, this is two campaigns handled by the same user(Hamphuz)and probably based on his agreements with the project company decided on 15 posts, he could manage a bounty tomorrow and it'll be 25 posts, there's nothing fixed, the weekly posts for those campaigns could even increase in the second week.
Believe it or not, if the campaign requires 25 posts weekly, people will keep applying and joining.
I think it all comes to how the manager and the owners of the project or business agreed upon certain terms.
Sure it is basic deal for business with signature campaign that should bring benefits for companies, managers, participants and the forum.
one thing I notice is these campaigns are paying with fixed $ rate instead of BTC amount/post/week
that would be slightly a disadvantage from participants perspective when BTC price is rising
Each types of payment rate have their own advantages and disadvantages for companies and participants.
In the long upwards trend of bitcoin, participants will feel more happily if they get paid in fixed amount of bitcoin. In contrast, their estimated payments in fiats might decrease if bitcoin price falls dramatically.