First and foremost, simplicity. The code is shorter and more manageable.
I don't know if functional program is SIMPLE as much as it is CONCISE.
I didn't see the word monad, fold, or reduce anywhere in the article. Is a functional programming language used with side effects and all? There's no acknowledgment of scenarios where managing the state the functional way can be unnatural, and require an intellectual hop. Or, alternatively, that this game in particular lends itself more naturally to the functional paradigm.