Yeah, there's gamers and then there's gamers.
Amazon isn't ranking based on gamers - it's ranking based on people who buy the game. Everyone knows Monopoly. Most folks were brought up with it as the example of a "good" board game, with little understanding of the meaning of that term, and Hasbro markets the heck out of it (and its other games).
Not to belittle your point, though: Corps are going to focus on what they can sell. Monopoly is easy and caters to the masses. Most folks aren't going to get or necessarily even enjoy the games ranked high on BGG, so they aren't going to get marketed to the masses...