Because Zork was written on the MIT Dynamic Modeling PDP-10. MDL was an important part of the software ecosystem on that computer, but Lisp wasn't. On the other MIT PDP-10 computers, Maclisp reined.
Was there any particular reason they did that, or was it just a random coincidence (that was the team that wrote it and the hardware they had access to was that particular machine and that particular machine ran MDL, otherwise, it would have been MACLISP)? Was there anything about MDL that helped with writing an adventure game?
