The concept comes from emergency survival gear. When you're building a 72 hour pack, a bag you make to survive an earthquake or other catastrophe, it's often recommended that you bring one item of personal comfort. This could be an inspirational text or a stuffed animal or even a little extra food. My car bag has an MRE (Meal Ready to Eat), even though I can't justify the weight. However, knowing I always have a hot meal, after who knows what, gives me comfort.
This one item is your exempt item, free from scrutiny of usefulness or weight. Putting a bag like this together is subjective, and it's easy to put in too much stuff. So your one item is an emotional crutch to allow you to be ruthless with the other items in your bag, which can number a couple hundred and if you're not careful, weigh more than you can safely carry. But you only get one.
That's the psychology behind your one game in inventory management. You can have that one game in your store that you're personally attached to. My game, for many years, was Tikal. I like this game. There are several ways to win. I like the theme, since I've been to Tikal and enjoyed my trip. I also tend to win it often. Who doesn't like a game like this? Unfortunately, it sold poorly, often just once a year.
Besides Tikal, I had a bunch of other games I was attached to, mostly games I had stayed late at the store to learn to play. I sacrificed personal time for these games, away from my family, so I could learn them and sell them better. Eventually, almost every game stops selling and you need to let them go, but personal investment makes that hard.
So the ONE game concept means you keep your Tikal, but you let the rest go. Hang on to your comfort game and know it's safe while you do the hard work of detaching from the rest. Don't let those games crush you with their worthless weight. Keep your Tikal and be ruthless with everything else, for your own survival.
Photo from Never Bored of the Boards