To attack the premise of the question: What are the arguments for substituting ß with sz?
A lot of things happened to German spelling since the appearance of the letter eszett. In particular, the aspects of German spelling due to which it made sense to use sz where we now use ß ¹ are long gone. So, while the eszett bears the letters s and z in name, this does not exactly refer to the letters s and z as they are used by modern German spelling, but to those letters as they were used centuries ago.
With other words: Apart from the linguistic and typographical history, the relation of eszett to s and z is by name only – the same way that the letter w is uu by name only in English. Thus the name somewhat is the only argument for substituting it by sz. In the same way, one might ask, why the letter ç is not substituted by cz, since that is its origin.
In modern German, the combination sz is only used in a few loanwords (e.g., Disziplin, Szene or lasziv), where it represents different sounds than the letter ß does (see also Toscho’s answer). On the other hand, the ss always represents the same sounds as ß, and only differs by indicating a different length of the preceding vowel: ß is preceded by long vowels, ss by short ones. (In the old spelling (before 1996), the difference is a little bit more complicated, but the argument is essentially the same.) Therefore, ss is the substitution that is closest to German spelling conventions.
¹ and which eventually gave rise to the sz ligature (more precisely: ſz), which rose to letter status and on which eventually the modern ß was based