It’s all in the presentation. The problem is that I read the intro & background before starting, in which they state that it took one guy (the professor, no less) a year to solve it, and others needed a whole semester. They also state that "only a basic understanding of mathematics is required."
Those statements created two false implications in my mind: first, that the solution involved mathematics (it doesn’t, unless you believe that counting is a math skill), and second, that reasonably intelligent people might need months to solve it (which is absurd, unless you believe that people can simultaneously be "reasonably intelligent" and not know how to count).
It would be interesting to create two separate webpages, one with just the game, and another with the game and the intro/background, and send some people to one and some people to the other, and see what the mean solution time is for each.