Only Satoshi will be able to give an accurate and exact reason why he left the community and remain unknown. It could be because he thought what he wanted to create has been created and no point of staying again as he wanted to make bitcoin to be for the public and not centralized to a certain degree. Because Satoshi remain anonymous is the reason some other people believe bitcoin is better and by that way, we can assume that anyone that contribute to bitcoin progress are also Satoshi.