I'm assuming partly inertia, partly the code density not being important enough to do this. To be clear, while you _can_ go OoO superscalar with a stack machine

I'm assuming partly inertia, partly the code density not being important enough to do this. To be clear, while you _can_ go OoO superscalar with a stack machine, it's more work than with an ISA that exposes the dependencies, like VLIW or EDGE. Don't take my word for it, design, model, simulate, and implement it yourself. It's a small matter of coding. EDIT: Reduceron is an example of a super-scalar stack machine, though not dynamically scheduled. It's very difficult to write code by hand though.