Here's why:
* Cantering and galloping are both gaits, but they're not directly sequential. A canter is a three-beat gait, while a gallop is a four-beat gait. A horse can transition directly from a canter to a gallop, and vice versa, without going to a walk.
* Horses can transition between gaits in several ways. They can go from a walk to a trot, a trot to a canter, a canter to a gallop, and so on. The specific transitions depend on the horse, rider, and training.
Imagine it like this: A car can go from 2nd gear to 4th gear without going back to 1st gear. It's the same with a horse's gaits.