Wasn’t it more to fit the constraints of the floppy disks? (Also, money.)
They say that they did a rewrite from the original version:
‘ZORK: The Great Underground Empire - Part I’ …was developed by the original authors based on their ZORK (Dungeon) game for the PDP-10. It features a greatly improved parser; command input and transcript output files; SAVEs to any device and file name; and adaptation to different terminal types, including a status line on VT100s. Note: this is not the FORTRAN version that has been available through DECUS. This version has been completely rewritten to run efficiently on small machines - up to 10 times as fast as the DECUS version.
…ZORK runs under RT-ll, HT-ll, or RSTS/E and requires as little as 20K words of memory and a single floppy disk drive. The game package, consisting of an RX01-format diskette and an instruction booklet, is available from Infocom, Inc., P.O. Box 120, Kendall Station, Cambridge, Ma. 02142."
