Lots of people out there have had bad lives with lots of trauma. The cult gives them a sense of certainty, a path to a good life if they just do x,y, and z. Others like to feel special, and having "The Truth" gives them that. They feel like they're in on some secret, knowing things the average person doesn't. Others are narcissists and the religion taps in to that because it's a very narcissistic religion. Still others hear about the end of the world and fear drives them.
Really, there are myriad reasons. It's all marketing and human behavior.
Me, I was just a kid. I know it's hard from a healthy place to understand why anyone would join, but the world has lots of unhealthy people in it. Some are bound to land in the cult.