Timing for the game loop and accessibility.

5 letters gives enough options without requiring the player to have access to a lot of guesses to have any hope of winning, and gives the game a short playtime.

It's built to be quick to play. You can play it on a commute, or dip in a few times throughout the day.

You wouldn't have that progress visible to players with a larger problem space. Tangentially, you'd also wind up having to lean more on "technical" (as in scoped to a particular domain, not technological) language in order to fill the list. That limits access to those outside of that field of study.

You’re just describing positive aspects of the game in its current state, not “anything other than 5 letters causes the game to fall apart”