why doesn't zaroff let rainsford know that his hiding place in the tree has been discovered ?

Zaroff thinks the hunt is too easy. He wants to extend it and leave Rainsford wondering where to go and hide.

but he knew that rainsford is like his accomplishments , yeah hunt in general to him is easy , but he is your rainsford , what is the other point ....

I'm not too sure what you are getting at here.