Usually country restrictions on media tend to have their origin in exclusive distribution agreements for certain countries or parts of the world. If you sold the exclusive right to distribute your video in say Europe to some third party back in the days before the internet you have no choice but to restrict access.

It is not done because it is the sane thing but because it is the legal thing based on contracts made in the times of outdated distribution methods.

Copyright is more complicated than it might first appear. There is a copyright on the music, the performance, the lyrics, but also the music video itsself, or even individual things used within the music video and all of these may be owned by different people each with particular license terms.

The default for IP holders is to be a twat. If they focused on making their portfolio available to the most people, there might pass an un-pinched penny through their claws. As a result, any kind of IP license must, apparently, be the maximum amount of pain and circumscription and sheer, bloody-minded pettiness it could possibly be.