Simply because it is easier to: one; break the horse in a western saddle, two; it is easier (for me) to ride in a western saddle, and three; teach the horse in an o-ring and go from little to no contact to lightweight contact on the mouth.
On another topic unrelated to the one above...
Sigh, collection is not only an English thing for all you people who keep saying "its so much harder to slow down and collect than to just extend like Western people".
First, English horses (if properly trained) are worked in extension first as well. Any horse should be taught extension far before collection is even dreamed of being worked on. No "green" horses are/can be taught to truly collect themselves. That comes much, much later, after essential muscles are more developed. And by collection I don't just mean that fancy headset you always see in Dressage; I have seen way too many horses with a headset that looks "pretty" that are hollow-backed as all getout. Headset can be forced; true collection (ie: hindquarters engaged and underneath the horse, neck elevated from the withers, head RELAXED at the poll, not HELD on the vertical, etc.) cannot be.
Second, Western riders teach their horses to collect too. Now before you jump down my throat saying I don't know what English collection is, etc, I am not a Western rider by any means. I'm 100% English, all the way. However, if you watch the reining horses and the cutting horses, and especially barrel racers, how do you think they execute those turns and moves with such speed and agility? Not by being hollow-backed and strung out I don't think. That is most definitely a result of a horse that has been trained to extend between barrels, and collect itself up VERY well going around those turns.
If properly trained, collection should come at about the same rate in either discipline. And yes, it takes years to teach a horse proper collection, whether you're riding English, Western, or whatever else you want to throw out there.
As for the OP; first train the horse in whichever discipline (Western vs. English) you feel more comfortable riding in, if you're going to be doing the training yourself.
As for which is easier to switch to.. it will most likely depend on the horse and the rider. Some people are much better at teaching English vs. Western aids/commands to a horse, just the same as some horses respond much better to the English style of riding and some prefer Western.