1. It's dull. You need something eye-catching to sell anything, flashing neon lights and unique, trademarked sounds will draw people to the machine.
2. It's unexciting. You need something more thrilling than just the ordinary pay-select-grab-walk away. You might for instance have to play a little flipper game with the coin you just put in. If you get the coin up the right ramp, you'll get your purchase.
3. It's predictable. Expanding on the flipper concept, if you lose the coin you'll get nothing, if you mess up you'll get the wrong product, and if you're really good you'll get a few more products for free.
4. It's unstimulating. Who says the product should come right out at the slot? The product will be transported through the machine, and you'll have to figure out how to keep it form falling in the trap, getting crushed by the anvil or eaten by the rat, using a nail, a piece of string and two balls of green yarn.
5. It's just too big. Why not make a personal vending machine, maybe even a portable one? Granted, the product will have to be scaled down to fit inside, but the convenience will more than make up for it.