The persecution became so severe that the apostles scattered throughout Judea and Samaria. (Go back and read again Acts 1:8.)
Philip is preaching in Samaria where crowds are listening eagerly to him and seeing the signs that he did (8:4-8). Peter and John returned to Jerusalem. And Philip is directed by a messenger from God to go to Gaza.
He is performing signs, drawing crowds, being praised, and baptizing. Philip is in a productive mission field. And God tells him to travel the wilderness road.
On the trip, Philip came across a court official of the Ethiopian queen who was returning from a trip to Jerusalem. He had gone there to worship, and when Philip saw him, he was reading from the prophet Isaiah.
We can speculate whether he had already read the part of Isaiah where eunuchs and foreigners are included in Israel's promise (56:1-8). [Tangent: We can further speculate on whether we ourselves have spent much time with that passage and whether we talk and act as if we believed it.]
The Spirit sent Philip over to speak to this foreigner. Philip responded to this command by running over to his chariot.
He asked him if he understood what he was reading. The Ethiopian replied, "How can I, unless someone guides me?" And he asked Philip to join him.
Some points to consider:
People who don't look like or who haven't been brought up like us may be sensing the call of God. God may be talking to us, and we ought to be listening.
If someone wants to understand scripture, and we're standing right there, we need to be prepared to step up to the need.
OTOH, scripture may not be transparent even to someone who has studied a lot. We need to look at the Ethiopian as a good example of someone who knew he needed instruction and was willing to admit it.