Canter is a very natural gait from which to learn collection because the horse can innately canter very fast or almost in place. Trotting is different, more artificial and usually more difficult, so it’s easier start in canter.
I teach collected canter in two ways. I start some horses with Exercise 1, “Schooling Through the Numbers,” and some with Exercise 2, “Changing the Balance Rapidly.” However, eventually, your horse needs to understand both.