The superantigen toxin is not produced by bacteria in every infection, and not all bacteria carry the genes for the toxin. There are various types of superantigen toxin, each with different levels of virulence.
Environmental factors (cues for gene expression) affect how much of the toxin is expressed by bacteria. This determines if and how much toxin is expressed and whether or not TSS will develop from a wound infection.
To answer your question, TSS is rare because not all wound infections meet the criteria for the expression of the class of superantigen toxin responsible for TSS.