It sounds like from what I've read MACLISP and MDL were side-by-side for a while at MIT and something of a department choice. MACLISP sounds like the serious effort and I read MDL as the "hip" or maybe rebellious upstart with a weirder sense of humor (it was called Muddle and spelled MDL to make it seem like an appropriately serious acronym), which would also make some sense that Zork originated in that allowed to be sillier language.

(Also, in reading other comments around here, I've learned there's a deeper connection in MDL to Scheme than I knew before, so I hadn't realized the Lisp/Scheme split even has ties to this "competition" of Lisp languages at MIT.)