Why did the chariot fail?

A major reason for the decline of the chariot as the tool for wars was that it was extremely expensive to maintain. Also, the horses had to be bred high enough to use and ride. Once horses were strong enough, the chariot was discarded as the latter needed flat and unobstructed grounds to operate.
Hence, the last known use of chariots in the battle between Romans and Caledonii in the Mons Graupius. It was in 84 AD.