Any number of reasons really, it could be there is a desire to keep his identity secret so that various government agencies, or people in a similar vein of that, malicious or not, have a hard time finding him. This could also be because Satoshi is actually a group, not a specific person and thus they use a generic name to prevent any sort of real affiliation. Alternatively, it is actually his real name and he simply disappeared for one reason or another and we just never knew what happened to him in the end. Which is possible.