Well, for me the point (essence) of promotions is for the casino to promote themselves to get more "new" gamblers to join the casino. Some of the promotions are there to get existing gamblers to gamble more. (Races/Championships/Tournaments/Challenges)

So, if the RTP is too high to pay for all of this, you will get new people to gamble and your existing gamblers will eventually get frustrated and they will leave. It turns into a cycle that will eventually "kill" the casino... because you will not keep the doors open, if "new blood" does not join the casino and the old gamblers leaving.