Because the Ukrainian church is part of the Russian Orthodox Church. They never said they were Russian or for that matter Ukrainian, if anything they are closer to Poles or Moldavians then Ruthians or Rus.
"Ukrainian church is part of the Russian Orthodox Church"
Wrong - the Lemko Churches and the Ukrainian Churches predate Muscovy - Kyiv was a "City of a Thousand Churches " when Moscow was still a podunk fur trading hamlet.