All horses know how to canter. A true lope is a slow, collected canter and takes some work by horse and rider, especially when first learning.
A quality lope is NOT easy for the horse. The hind end is tucked, rear legs reaching, back is up and round, shoulders are elevated, neck and head are relaxed and low. Very few highly bred WP horses come born with a good lope, and even fewer "normal" stock horses are born with one.
If you only slow the canter, you end up with a horse tipped on the forehand and strung out behind, like so many of the WP hoses you see in the lower level of competition (and even some in the higher levels).