Why did riku submit to the darkness

Riku felt that the dark was more powerful.


Actually, he only did that in Kingdom Hearts 1. When all that is over, he sees he needs to fight the darkness, but when he's fighting Roxas he sees that the only way to defeat him is to give way to the darkness in his heart. He does so finally after a loss against Roxas and he becomes more powerful. He beat Roxas and worked with DiZ in order to wake Sora. This may not be the answer you wanted, though...