Why doesn't that portable cart inside the supermarket slide down on the elevator

Because the front and rear wheels are on different gauges. The supermarket inclined elevator is made of a bunch of parallel grooves. The wheels of a shopping cart look like an H from the front, and these two elongated side pieces nestle into the grooves of the inclined elevator when they get on. The front and rear wheels of the cart have different widths of the H, and the front wheels fit into the grooves.

Main Advantage:

Because there is a notch on the top, and the wheel of the cart has a bulge, and then the depth of the notch, is greater than the height of the bulge on the wheel, and the width of the cut is also greater than that of the latter in the former, but also because there is a rubber on both sides of the wheel to do the brakes, which sums up the fact that the cart is suspended in the elevator, and only the rubber is as a brake with the Elevator contact, so when the elevator stops moving, the cart is not pushed.