There has been bad blood between UPS and Canada Post ever since UPS came to Canada and found a way around the postal regulations by using modified Checker Cabs to make their deliveries instead of conventional trucks, which somewhere in the regulations was prohibited except for Canada Post.
Also, when you use USPS, the package is handled by Canada Post on the Canadian side, so the bureaucrats are happy with that arrangement. All UPS shipments have to go through a broker, so they are shipped again after passing through the broker. Most if not all USPS shipments do not.
Since the last Canada Post strike, I have reverted to always sending USPS Priority Mail as it can take up to 5 weeks if you just use International First Class.