If I'm not mistaken they never open a slot for low rank member ever since before and they continuously running their camp with hero/legendary rank members.
Yes, you are right, in this case Wolfbet want to keep signature campaign participants keep on the line to build their brand and trust. Well, signature campaign not the only ways from Wolfbet to run a promotion, if they are happy with current marketing results, maybe hard to see campaign slots for lower rank. IMO, 1 Legendary member will be better than 3 Full Member rank to build a brand and trust.