Cows can walk down stairs, but it is extremely difficult for them due to weight distribution, how their knees bend and the positioning of their hips. All of these factors can easily cause a cow to tip forward and tumble down stairs.
Stairs are a human invention designed for the legs and walking abilities of people rather than cows. Cows can climb stairs because they can push up with both sets of legs, keeping their body weight balanced. Going down stairs pushes the majority of the cow's weight to the front of her body. Because of the way the cow's knees and hips work, the cow cannot compensate to even the distribution of her weight while walking down stairs.