Understood. My situation is unique in that I hold dual citizenship and living/working in either country isn't an issue.
I've inquired with the MTO and they don't have an answer. An American moving to Canada can easily exchange a regular drivers license for an equivalent license. But exchanging a CDL-A for a AZ doesn't apply. And I simply wonder "Why not?" Driving a truck in Canada isn't different than driving a truck in the States.
I gave up my CDL-A and started from scratch through my Canadian employer to obtain a BZ. When I returned to Oklahoma five months ago I gave up my Ontario BZ and was reissued my Oklahoma CDL-A since they will do that provided it has been less than 36 months since giving up an Oklahoma CDL-A.
Anyways, because no government agency I've talked to has been able to provide a concrete answer to this I guess I'll always wonder "Why not?"
As an aside to your Texas residency example... my residency is Oklahoma but when I take my time off I park my truck at a USX facility in New York and drive my car across the border to Ontario where my wife lives.