[silvestrov]

Only encoded characters take up more space and you don't have to escape: quotation mark, semicolon, comma, colon or backslash.

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.

[SCLeo]

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.

[danachow]

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)?

[nemetroid]

At equal printed sizes, QR codes with less information are much easier/more forgiving to scan.