Why did the Holy Spirit tell Philip to go near the Ethiopian's chariot?
Because God in His sovereignty had an intrinsic interest in this particular Ethiopian, intending to bring him to a knowledge of salvation. The Spirit directed Philip as the human instrument, not an angel or elite scholar, to explain Isaiah 53 to a man who admitted he could not understand it.
Les explains that this event displays the sovereignty of God, who is in control of every detail He wants to control. God had an intrinsic interest in this situation, so the Holy Spirit directed Philip to this particular person. The Ethiopian had been to Jerusalem to worship and was returning in his chariot, reading Isaiah the prophet, specifically Isaiah 53. Philip ran to him, heard him read, and asked, 'Understandest thou what thou readest?' The man replied, 'How can I?' Les maintains that God did not leave His Word under the control of angels or highly educated seminarians, but with the leading of the Holy Spirit placed it in the hands of anyone willing to handle it. For those who will be made aware of the plan of salvation, God does not send an angel or a super-educated person; He uses common, ordinary people. It normally takes a human instrument to lead someone to salvation, and Philip was that human instrument.
Then the Spirit said unto Philip, Go near, and join thyself to this chariot.