There is no reason. We identify things in terms of words, not positions on the stack. We think "multiply first vector's y coord by second vector's z coord", not

There is no reason. We identify things in terms of words, not positions on the stack. We think "multiply first vector's y coord by second vector's z coord", not "multiply item 15 on the stack by item 12 on the stack". Thus, stack-based languages lose out in usability and are really just a curiosity without much practical applications.