The issue with international is that:

i) AC's aircraft has a small J cabin compared to other airlines such as UA.

ii) Many AC oversea stations do not use their own system so this makes day of departure upgrading impossible since you can't verify amount of miles available.

iii) There won't be much seats left, if any at all, after FFers upgrade using their CERTs.

iv) With AC having smaller J cabins, you want to ensure that all the upgrade seats are not taken by those using points so those with status can't upgrade. If you only allow upgrade on day of departure, you run into problem (ii) above.