Nothing but the desire to be the centre of attention. If you dig far into the internet to find out who Satoshi is, you'll find a website that mentions one of these identities as Satoshi Nakamoto. What else do they require if not attention of publicity? People impersonate not just anyone, but the resourceful individual who has created something valuable that many people are drawn to. There are two motives for this: either to gain popularity or to scam people into believing they are one and thus believing in what they urge them to do.