The story in Acts 8 of the Ethiopian eunuch actually begins a thousand years earlier, at the time of King Solomon. We read that Solomon was visited by the Queen of Sheba, with a large entourage. At that time "Sheba" included both what is now Yemen and more territory on the other side of the Gulf, on the Horn of Africa, including part of Ethiopia. It says in I Kings 10 that Solomon gave the Queen "all that she asked". It is believed in Ethiopia that this included a son, who became King Menelek of Ethiopia. There has been a community of believers in the God of Israel for many centuries, which may date back to this time. There is also a tradition that Menelek made a trip to Jerusalem to visit his father Solomon, and learn about his God. In any case, there was trade and contact between the northeast part of Africa and the eastern coast of the Mediterranean.
The Ethiopian community of believers, one name for which is the Falasha, have Scriptures in Geez, their ancient language, and also perform many of the sacrifices and feasts as described in the Old Testament. (Many of them have emigrated to Israel in this century.)
There is also a church in Axum, the ancient capitol of Ethiopia, St. Mary of Zion, which claims to have possession of the Ark of the Covenant. The traditional story is that when Menelek visited Jerusalem, he had a replica of the Ark made and brought home the original. Another suggestion is that during the period that the Temple in Jerusalem was being used for pagan worship, the Ark was smuggled out to a Temple known to have been built at Aswan, on the upper Nile, on an island known as Elephantine. Papyri in Aramaic dating from the 5th century BC have been found describing a military outpost and Temple observances. This community was disappeared, and there are legends of a holy object moved up the Nile to Lake Tana in Ethiopia. In any case, there have been Old Testament traditions and worship in Ethiopia for many centuries.
This makes the presence of an Ethiopian official visiting Jerusalem for Passover, who was familiar with the scriptures, and eager to learn not surprising. Add to this the instructions given to Phliip to find this person and give him the "rest of the story", and we see another case of one person in the right place at the right time with the Word of God, being able to be a seed planted that became one of the oldest Christian communities in the world.