Although cutting down the clone was not enough to cause the true fall of that deity, Zhou Tian knew that after he had killed that deity, even if the deity would not fall from his attack, it had definitely suffered severe damage. He believed that, unless there was an accident, for a considerable length of time, his opponent would not be able to recover and under those circumstances, it was unlikely that the deity would come out to fight against him again anytime soon.
After killing one deity, Zhou Tian's pressure naturally reduced significantly. However, although killing one deity had an effect, it wasn't enough to satisfy Zhou Tian at that time.
Zhou Tian's goal was to completely eliminate all his opponents in front of him. Thus, merely killing one opponent couldn't possibly satisfy him. Therefore, when Zhou Tian's attack ended, without a second thought, he launched the same attack on another deity.