Taken from Wikipedia.org:

Both Crowther's and Woods' version were designed to run on the PDP-10, enabling certain features unique to the platform. The PDP-10 architecture was 36-bit, with each word able to store five 7-bit ASCII characters. The game's FORTRAN code compared player's commands with its vocabulary, but using only the first five letters of each English word. Unfortunately, this limitation was silently evident to the game player too, and adversely affected gameplay (“north” would be equivalent to “northeast”). Hence, Woods added the five letter limit notes to Crowther's original game instructions. The PDP-10 also implemented application checkpointing which allows saving and restoring of the state of the entire program, instead of a more traditional save file. Both these features made it difficult to directly port the code to other architectures.

Colossal_Cave_Adventure#Later_versions