It's a little more complicated than that, and I can't fully explain it in an HN comment. I'd have to give you a crash course in compiler design. But the TL;DR is that if you're building a custom language, Lisp gives you the AST (abstract syntax tree -- look it up if you don't already know) for free. You can also sometimes leverage parts of the Lisp compiler depending on what machine you're targeting, though for the 6811 that was not the case.