The container traps the wrong thing
Fresh fries are crisp because their hot, dry outer layer has little moisture. Inside a room-service box or covered dish, escaping steam hits the cooler lid and condenses into water. That moisture settles back onto the fries, softening the crust that gave them their crunch. A tightly closed container slows heat loss, but it also holds in the steam; a vented one lets moisture escape, while allowing heat to leave faster.
The trip keeps changing the balance
Fries are small, so they have plenty of surface area relative to their volume. That makes them quick to cool once they leave the kitchen. On the way upstairs, heat passes from the fries into the container and surrounding air, while condensation continues to dampen the surface. A cart may keep food together and protected from drafts, but it cannot recreate the dry, high heat that keeps fries crisp straight from the fryer.
The result is a double change: the fries lose warmth as they travel, and their crust absorbs moisture along the way. Keeping them hot and keeping them crisp are not quite the same problem—a sealed package helps with the first, but can work against the second.
