to answer that we first have to figure out which set of curves he had access to (in order to choose from) back in 2008. i couldn't find any list sorting the curves chronologically but the safecurve website has the date of their paper in its table here: https://safecurves.cr.yp.to/

assuming Satoshi was looking at most popular/used curve and also a curve that has a standard defined by NIST (similar to the hash algorithm that was the most popular and had standard by NIST) the choices weren't really that big.
- Curve25519 released in 2005 but it wasn't popular until 2013
- brainpool curves' standards seem to be from 2010
- the rest of the dates are from years after 2008

that leaves NIST curves. so now the choice is limited to a much smaller number of curves.
if we consider size of them (192, 224, 256, 384, 521) and the recommendation by SEC about the fact that 256 bit curve offers a good security for many years and the fact that it matches the hash function (SHA256) makes that group a good choice.

now the list has 2 curves in it: secp256k1 and secp256r1
between these two since the first one is a Koblitz curve and it can be optimized more than the random curve, the choice is obvious.