A more interesting question is why he went for secp-256k1.
From the wiki:
Also, unlike the popular NIST curves, secp256k1's constants were selected in a predictable way, which significantly reduces the possibility that the curve's creator inserted any sort of backdoor into the curve.
That could be a reason... reality of the situation is, we will never know the true intentions of satoshi. He HAD to choose something, we would have endlessly discussed why he did choose those constants and not others no matter what. Same goes for 21 million limit, 1 MB blocksize, 10 minute blocktimes, and so follows.