Why do Buddhists believe in "reincarnation" but Athanasius is only point to one definite incarnation of Jesus Christ...
In Upanishadic religions (Buddhism, Hinduism, Jainism) "Re-incarnation" refers to rebirth in a new body after death. This is often called the "hell of rebirth" or "samsara", a cycle of re-incarnation, death, and re-incarnation that must be escaped for one to escape suffering.
St. Athanasius is using "incarnation" in the Christian meaning, that is, the birth in-time of the Word of God as a real human being. Christians do not believe in Samsara.