Interestingly, pentacostalism started after Russell's international BS movement, I think 1906? Now there are 220 million followers, Why the fast growth?
Pentecostalism appeals to people who already believe in spirits, snd in some countries where animism is a thing, it does a good job of blending it with christianity. Get a charismatic pastor who can whip up emotions in people, and it will be interpreted as a "spiritual experience."
It appeals to people's pride, because they convince people that they have a specific gift that others outside the faith don't have: "hey, you've been given a gift by the holy spirit! You have to use that gift for the Lord!" (This is similar to jw's "God sees special something in you and that's why he's chosen to reveal truth to you while the world remains ignorant!")
Fun fact: Originally Pentecostals who had the "gift of tongues" were sent out to foreign lands to preach in native languages. Unfortunately they found out that no one on earth understood what they were saying and that they would have to study the languages like everyone else. Thereafter they began to believe they were speaking in "tongues of angels."