Because the Spirit is leading many into a "deluding influence".
So the Holy Spirit is leading people into error? I see, an evil Holy Spirit and a good Holy Spirit sort of like the Gnostic heresy that there was an evil god in the Old Testament and a new good god in the New Testament. People who claim the Holy Spirit would lead people away from Christ are themselves listening to whatever spirit is hanging in a nearby tree, not to the Holy Spirit.
The Holy Spirit doesn't lead people into error. Anyone who thinks that's possible is in essence denying that the Holy Spirit is part of the Trinity and therefore by definition is not Christian.