I would guess it is because their belief on the acquisition of the Holy Spirit is a separate experience from salvation or Baptism. I don't know how it is exactly tied to that, but that's my best guess.
The Pentecostal congregation that I found Christ in, they did not believe they are 2 different acquisitions. One acquires the Holy Spirit with the new birth. The Holy Spirit then dwells in your heart. The baptism of the Holy Spirit is the release of the Spirit through your body. In other words, with this release is the river of living waters that Jesus spoke about.
Someone said that if God does not complain that we don't have the baptism of the Holy Spirit, then he was not concerned about. God does not complain. He informs. He tells us that if we remain in our sins we will die, but he does not complain, nor make us repent. He informs us not to minister until we receive the power from on high. He does not make us take the baptism of the Holy Spirit.
In regards to two separate experiences of the Holy Spirit we see them here. After his resurrection he appeared unto the disciples.
Joh 20:20 And when he had so said, he shewed unto them his hands and his side. Then were the disciples glad, when they saw the Lord.
Joh 20:21 Then said Jesus to them again, Peace be unto you: as my Father hath sent me, even so send I you.
Joh 20:22 And when he had said this, he breathed on them, and saith unto them, Receive ye the Holy Ghost:
It was at this point they were born again. They already believed he was Christ who died for their sins and rose again, since he was standing in front of them.
But the baptism of the Holy Spirit happened 10 days after his ascension on the day of Pentecost, and that was when they spoke in tongues.
This is why there are two experiences of the Holy Spirit.