That's worth its own thread, don't you think?
Part of the answer lies in government encouragement (tax breaks), little/no regulation or requirements for church leaders/minister/pastors (no test, no training whatsoever required, just a valid ordination certificate). Some of them are just ignorant and uneducated. Some of these people are deeply unhinged, and very frightening. They make Karl Rove look like a choir boy.
All it takes to establish a legal church in the US is a person with an ordination certificate and two other persons of legal age to be secretary and treasurer, provided they meet monthly and keep minutes. All I need to start up "Our Lady of the Perpetually Peaceful Pixels" is the two other people. Which in this highly conservative area would be like finding a live dinosaur that talks and does the hula.
I'll take a stab at answering your question, though understand I am coming from outside mainstream and charismatic religion and therefore my observations may be as through a dark glass.
Mainstream Christianity (Catholicism, Church of England, Lutheran, Methodist and so forth) basically say, "Worship Christ, pray to him and he will tell you what to do."
Charismatic, or Evangelical, Christianity says, "Worship Christ, send him your prayers, but ask me what to do." And any time a believer is in doubt he or she can call Rev. Jones across town and get an answer right away." It's the instant gratification, plus the relief of not having to actually exert one's self to actually think about what it is they're doing. Some people prefer to be sheep. I've never understood it.
Another factor is the social interaction. In many of these evangelical churches, members address one another as "Brother" and "Sister", Brother Jones, Sister Smith. This creates a false sense of belonging, as to a large family, at least for the duration of the service. There is also a lot of hand-shaking and hugging. (I've been to a few of these.) In our fragmented, paranoid society, we often lose sight of the fact that human beings need to be touched, albeit carefully, and to belong to some kind of tribe. The Evangelical church offers that subtly in its structure and rituals. It also offers people whose ability to lead themselves is impaired (alcoholics, drug addicts, obsessive-compulsive, abusive, etc) a way out of their dilemmas: "Come, follow me, do as I say and you'll be cured." And some of them recover. People in precarious straits are vulnerable to that.
Evangelical Cristianity tends to split society into "them" and "us", usally along racial lines, and what often occurs is that the "Hate" energy of the group is directed at "them" and the "Love" impulses toward "us". It boils down to a perversion of the commandment, "Love thy neighbor as thyself." In the hands of many, not all, but many Evangelical congregants, this becomes "Love thy neighbor who belongs to this church as thyself; if he doesn't belong to this church you can dump fertilizer in his yard."
That's by no means all there is to it, but perhaps enough of an overview to give you a little insight. There is a lot of focus in Charismatic services on the rapture and the second coming, and a lot of promise of material wealth then. This has a lot of appeal to people who have almost nothing in the here and now. And we have a lot of those.
"God made a lot of poor people, and it's no shame to be poor, but it's no great honor, either." --Tevye, Fiddler on the Roof