God appointed Joshua to succeed Moses because he was a man in whom was the spirit. This is stated in Numbers 27:18 (RSV):

"And the Lord said to Moses, ‘Take Joshua the son of Nun, a man in whom is the spirit, and lay your hand upon him.’"

This verse clearly shows that Joshua was chosen because of the spirit of wisdom and leadership in him, making him the rightful successor to lead the Israelites into the Promised Land.