Why? Because he was familiar with elliptic curve cryptography?

My impression is that Satoshi was not an expert in anything. He was a generalist. He knew just enough about cryptography, programming, economics, and peer-to-peer networks to make Bitcoin.

He needed the cryptography knowledge to know the properties of ECDSA and hashing functions; programming to actually write the code; economics to know there was actually a need for private, unrestricted money; and peer-to-peer networks to know that this was also needed to make Bitcoin robust.

He has a profile on p2pfoundation.org. He has posts from 2009. It seems like it's him (I don't think anyone knew anything about Bitcoin back then, so it's doubtful that it's someone faking him; they'd be faking a nobody (or so they'd think)). http://p2pfoundation.ning.com/profile/SatoshiNakamoto

Well... do you have the t-shirt to prove this?