John Paul II wasn't allowed to visit russian catholics by ortodox Aleksyei.==
There are almost no russian catholics. Those who catholic in Russia usually are not russians.
Pope wasn't allowed because he refuse to follow rules for visiting religious leader which are installed in Russia.
Russia has her own christian ordtodox church.
SO if the leader fo different church want to come in Russia he has to come under invitation of patriarch.
Partiarkh Alexus said that Pope refused to regulate some questions which are between 2 churches. So he won't invite him to Russia until it would be regulated.
So as I see it that the stubborness of Pope stopped him to visit Russia.