The most accurate response to the question about why the focus was primarily on Christian art during the period from the start of Christianity in Europe to the Middle Ages is:
All art created during this time was related to and centered around religion—specifically, Christianity.
This reflects the significant influence of Christianity on culture, society, and artistic expression during this time, as the church played a central role in everyday life and artistic patronage. While there were certainly other factors at play regarding the creation and preservation of art, the dominant theme of religious devotion and representation is the clearest and most encompassing reason for the focus on Christian art during this period.