Because the semantics make no sense or are difficult to define. Beans and maps associate a value with a property name. Lists and arrays do not. And it hardly makes sense to set things into a collection by index. What does it mean to set a value into the 213rd index of a 100-element array?
In all my years of writing web apps, this is not a need I have ever had and apparently the framers of the JSTL saw no need for it either.