Typically, restaurants get their soda syrups in plastic bags, but Coca-Cola does something different for McDonald's. The fast food chain gets its Coke syrup delivered in stainless steel tanks. According to the New York Times, the material keeps the soda fresher, and your tongue can taste the difference.
Because it lets more carbonation hit your tongue and makes the soda taste better. “That's why it's better at McDonald's.” The McDonald's US website confirms the straw thing, and also adds that the Coke syrup is pre-chilled before it's added to the drinks machines; adding to the amazing flavour.