Because the fraction represents a rewrite rule, which is the fundamental unit of computation in this language. Instances of the denominator factors are replaced

Because the fraction represents a rewrite rule, which is the fundamental unit of computation in this language. Instances of the denominator factors are replaced with instances of the numerator factors. Some people read x/y as a rewrite already (like programming language nerds). Others don't, and it makes sense to me to use a more intuitive, directed symbol to denote the action. FWIW, `>` has not one but many meanings across programming languages.