As far as I know, Roobet does not conduct any marketing campaigns outside the forum, and all (or most) of its strategy depends on the bitcointalk community. I agree with you in suggesting that they can diversify advertising plans, but not by changing a successful one.
Roobet actually does advertise their services through social media sites like Twitter etc and don't just depend on Bitcointalk. Also, no one apart from their team knows whether their marketing forums within this forum have succeeded or failed.
Personally, I am just happy that they consistently keep hosting art competitions regardless of whether they are succeeding or failing.