The same reasons perfectly intelligent people join and remain in actual cults.
Honestly, there are so many factors involved in it's hard to enumerate them all in an HN post.
For those who are intrigued by actual cults, I strongly recommend the podcast "Cults" on the ParCast network. It can come off as a little dry at first, but stay with it, they not only tell the story of the cults they cover they also reference real, mainstream psychological researching explaining the motivations of the cult leaders and members. It will change the way you think about all organizations, not just fanatical religions.
Whenever I hear about a particular technique that is supposedly backed by evidence I wonder what specific protocol was used in the studies supporting it. I also wonder how many studies have been conducted and for how many years.
For those of you who want an actual evidence-based practice check out Mindfulness-Based Stress Reduction (MBSR). It's the most frequently studied protocol and it's been studied for decades. When you read stories about studies showing how meditation does XYZ and ABC, it's almost always MBSR that the researchers are using.
"Meditation" is a huge category, there's a lot of stuff in there. Some of it probably works, a lot of it doesn't (other than the placebo effect).
What you think of as "meditation" may not be what actually has been researched in a given study.
Of course, all of the above is only applicable to those of you who want to pursue evidence-based techniques. If you're just satisfied with trying stuff you read about on random blogs and HN posts then go ahead and try anything: prayer, astrology, witchcraft, etc - you'll probably realize a little positive benefit as long as you genuinely believe it will work.