In Orthodox Christianity, the jurisdiction of parishes within a region is governed over by an archdiocese, which is presided over by a bishop. A Patriarch is basically a bishop that oversees multiple archdioseses and their bishops.

The chain of bishops is an integral part of keeping track of apostolic succession. So being under the authority of a canonical bishop is essential.

The issue here is that the Ukraine government wants to put into place state-approved self-proclaimed priests and bishops, instead of those that are canonically ordained and recognized as legitimate by the Church. A breach of this canonical hierarchy is also a breach of communion.

Basically, Ukraine wants to put puppet cosplayers in place of actual legitimate canonical clergymen.

Moscow Patriarchate has pretty much always had canonical jurisdictional authority over Kievan Rus, now known as Ukraine.

Zelensky deliberately conflates the Russian State with the Russian Church, even though they are very clearly distinctive entities.

Even before this war broke out, Zelensky and the Ukrainian government were actively calling for the persecution of Ukrainian Orthodox Christians that were under the Moscow Patriarchate.

For Zelensky, it's not simply a war of politics, but a war against Russian culture and spirituality.

Zelensky is really no different in his spiritual character the bolsheviks of old whom sought to erase any semblance of Orthodox Christianity and Tsarism off the face of the earth in Russia. [His very tribe, the Jews, were also responsible for /that/ violent revolution.]