The normal New Testament practice was for repentance for sins, believing in Jesus Christ for salvation, being baptised and receiving the Holy Spirit to go together as a single act.
(See One Baptism)

If anyone does not have the Spirit of Christ he does not belong to Him. (Rom 8:9). Hence the disciples of John the Baptist at Ephesus did not have the Holy Spirit because they were not Christians. John had prepared the way for Jesus - on being told about Jesus they were baptised in His name and received the Holy Spirit when Paul laid his hands upon them. (Acts 19:1 - 7)

In the case of the Samaritans they had believed and been baptised but the Holy Spirit had not yet fallen on any of them. This is the only passage in the New Testament where such an expression is used. It implies that a normal part of the sequence of events had not occurred. The apostles came and laid hands on them, and they received the Holy Spirit. (Acts 8:5-17)

Why was there this unusual delay? Perhaps to ensure that the church was totally unified. The schism which existed between the Jews and the Samaritans could not be allowed to enter into the church. The coming of the apostles to Samaria and the Samaritans receiving the Holy Spirit when the apostles laid hands on them was the same experience that many of the Jews must have had in the Jerusalem church. Jew and Samaritan were united in Christ.

See Appendix for a Comparison between John's Baptism and the one Administered by Jesus.