I could only guess.

Satoshi Nakamoto's goal was to create a decentralized system. In order to achieve complete decentalization, no figure/person should be on the center of it. If he the inventor and creator introduced himself to the community, whether he likes it or not, he will by default be at the center of it and will become the de facto leader of Bitcoin, gaining himself power and influence which goes against what he wants to create.

My other guess is that, since this is a completely new and pretty much a "controversial" technology with no backing by the government, he probably feared that the the authorities will be after him after some time. To prevent this, he opted to hide himself from the community right from the beginning.