Why are there different saints in the Orthodox Church and the Roman Catholic Church?
When the Eastern and Western parts of Christendom were united, which was roughly up to the mid-eleventh century, the two Churches shared the majority of
saints, especially the saints of Holy Scripture, such as the Apostles, as well as the great fathers and teachers of the Church, such as St. Athanasios, St. Leo of Rome, and St. Basil, and in addition other saints of the early Christian centuries. But even during this period, many saints were of local significance and not universally commemorated in both East and West. After the separation of the two Churches, however, each Church developed its own list of additional saints.
It should be noted that the Orthodox Church claims to be the One, Holy,
Catholic, and Apostolic Church, as we profess in the Creed, which has retained
its catholicity and unity, despite the separation of Western Christendom. The
Western Church came to be called the Roman Catholic Church and from it many
Protestant Churches separated and then were themselves divided. The
Orthodox Church is a family of regional Churches, Greek, Russian, Romanian, Syrian, Serbian, Georgian, etc., united in doctrine and sacrament, but independent in administration. These Orthodox Churches, too, while sharing the majority of Orthodox saints, also commemorate other saints significant to their own particular history and traditions, such as St. Vladimir in the case of the Russian Orthodox Church.