In countries that do not use English as the main language, it is fairly common to have non-English SSIDs. URL encoding is incredibly inefficient when encoding those characters.
QR code is not particularly dense (like compared to something like a hard drive) - why waste space that could not be put towards more redundancy (error correction)?
So I think the difference is small. QR codes can contain quite a bit more information than what's needed for WIFI name and password.