Elliptic curve and "vanity" public keys
Maxwell's vanity public key is a result of how the generator of the secp256k1 was chosen; as explained by Maxwell himself.

For some reason, the generator $G$ is the double of the point:

x = 0x00000000000000000000003b78ce563f89a0ed9414f5aa28ad0d96d6795f9c63, 
y = 0xc0c686408d517dfd67c2367651380d00d126e4229631fd03f8ff35eef1a61e3c


which is exactly Maxwell's vanity public key. Its private key is 0x7fffffffffffffffffffffffffffffff5d576e7357a4501ddfe92f46681b20a1 which is $1/2$, i.e. the inverse of $2$ modulo the curve order $n$. This is also equal to $(n+1)/2$.

The big question is: why was the secp256k1 generator chosen like that? I have no idea. But note that it's not difficult to find a generator with a lot of leading zeroes.